Control apparatus, control method, and engine control unit

ABSTRACT

A control apparatus which is capable of compensating for a control error properly and quickly even under a condition where the control error is temporarily increased e.g. by degradation of reliability of the detection results of reference parameters other than controlled variables, thereby making it possible to ensure a high accuracy of control. An air-fuel ratio controller of the control apparatus calculates an air-fuel ratio error estimated value and an error weight, calculates an modified error, calculates a basic lift correction value such that the modified error becomes equal to 0, calculates a lift correction value, calculates corrected valve lift by adding the lift correction value to valve lift, calculates a first estimated intake air amount for feedforward control of an air-fuel ratio according to the corrected valve lift, calculates an air-fuel ratio correction coefficient for feedback control of the air-fuel ratio, and calculates a fuel injection amount according to these.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus that calculates acontrol input based on a value calculated by a feedback control methodand a value calculated by a feedforward control method, to therebycontrol a controlled variable using the control input, a control method,and an engine control unit.

2. Description of the Related Art

Conventionally, as a control apparatus of this kind, the presentassignee has already proposed a control apparatus disclosed in JapaneseLaid-Open Patent Publication (Kokai) No. 2005-315161. This controlapparatus controls the air-fuel ratio of a mixture in an internalcombustion engine as a controlled variable, based on a fuel amount as acontrol input, and is comprised of an air flow sensor that detects theflow rate of air flowing through an intake passage of the engine, apivot angle sensor that detects a valve lift, a cam angle sensor thatdetects the phase of a camshaft for actuating an intake valve to openand close the same, relative to a crankshaft (hereinafter referred to as“the cam phase”), and a crank angle sensor. Further, the engine includesthe intake passage having a large diameter, as well as a variable valvelift mechanism and a variable cam phase mechanism as variable intakemechanisms. In the engine, the valve lift and the cam phase are changedas desired by the variable valve lift mechanism and the variable camphase mechanism, respectively, whereby the amount of intake air ischanged as desired.

In the above control apparatus, as an intake air amount, a firstestimated intake air amount is calculated in a low-load region based onthe valve lift and the cam phase, and in a high-load region, a secondestimated intake air amount is calculated based on the flow rate of air.In a load region between the low-load region and the high-load region, aweighted average value of the first and second estimated intake airamounts is calculated. This is because in the low-load region where thereliability of the second estimated intake air amount is lower than thatof the first estimated intake air amount due to the large diameter ofthe intake system of the engine, the first estimated intake air amounthigher in reliability is employed, whereas in the high-load region inwhich occurs a state opposite to the above state in the low-load region,the second estimated intake air amount higher in reliability isemployed. Further, a basic fuel amount is calculated as a value for usein feedforward control of the air-fuel ratio based on the thuscalculated intake air amount, and an air-fuel ratio correctioncoefficient is calculated with a predetermined feedback controlalgorithm such that the air-fuel ratio is caused to converged to atarget air-fuel ratio. A final fuel amount is calculated based on avalue obtained by multiplying a basic fuel amount by the air-fuel ratiocorrection coefficient. Then, this amount of fuel is injected intocylinders via fuel injection valves, whereby the air-fuel ratio isaccurately controlled such that it becomes equal to the target air-fuelratio.

According to the above-described control apparatus, when detectionsignals from the pivot angle sensor, the cam angle sensor, and the crankangle sensor drift due to changes in temperature, for example, or whenthe static characteristics of a variable valve lift mechanism and avariable cam phase mechanism (i.e. the relationship between the valvelift and the cam phase with respect to the control input) are changed bywear of components of the two variable mechanisms, attachment of stain,and play produced by aging, the reliability of the results of detectionby the sensors lowers, which can result in a temporary increase in thecontrol error of the air-fuel ratio. More specifically, when the firstestimated intake air amount ceases to represent an actual intake airamount, and deviates from the actual intake air amount, there is a fearthat the fuel amount cannot be properly calculated as a control input inthe low load region where the first estimated intake air amount is usedas the control input. In such a case, the difference between theair-fuel ratio as the controlled variable and the target air-fuel ratio,that is, the control error increases. Although the control error can becompensated for by the air-fuel ratio correction coefficient in a steadystate since the air-fuel ratio correction coefficient is calculated withthe predetermined feedback control algorithm, it takes time before thecontrol error is compensated for by the air-fuel ratio correctioncoefficient. Therefore, e.g. when the control error temporarilyincreases, the accuracy of control is temporarily degraded, whichresults in unstable combustion and degraded combustion efficiency. Theproblem described above is liable to be more conspicuous in a transientoperating state of the engine.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a control apparatus,a control method, and an engine control unit which are capable ofcompensating for a control error properly and quickly even under acondition where the control error is temporarily increased e.g. by thedegraded reliability of the results of detection of reference parametersother than controlled variables, thereby making it possible to ensurehigh-level accuracy of control.

To attain the above object, in a first aspect of the present invention,there is provided a control apparatus for controlling a controlledvariable of a controlled object by a control input, comprisingcontrolled variable-detecting means for detecting the controlledvariable, reference parameter-detecting means for detecting a referenceparameter of the controlled object other than the controlled variable ofthe controlled object, target value-setting means for setting a targetvalue serving as a target to which the controlled variable iscontrolled, and control input-calculating means for calculating a firstinput value for feedforward control of the controlled variable,according to the reference parameter, using a correlation modelrepresentative of a correlation between the reference parameter and thefirst input value, calculating a second input value for performingfeedback control of the controlled variable such that the controlledvariable is caused to converge to the target value, with a predeterminedfeedback control algorithm, and calculating the control input based onthe first input value and the second input value, wherein the controlinput-calculating means comprises error parameter-calculating means forcalculating an error parameter indicative of a control error to becompensated for by the first input value, based on the controlledvariable and the target value, influence degree parameter-calculatingmeans for calculating an influence degree parameter indicative of adegree of influence of the reference parameter on the error parameter byusing an influence degree model representative of a correlation betweenthe influence degree parameter and the reference parameter, correctederror parameter-calculating means for calculating a corrected errorparameter by correcting the error parameter by the influence degreeparameter, model-modifying means for modifying the correlation modelaccording to the corrected error parameter, and first inputvalue-calculating means for calculating the first input value using themodified correlation model.

In the case of this control apparatus which calculates the first inputvalue for feedforward control of the controlled variable according tothe reference parameter, using the correlation model representative ofthe correlation between the reference parameter and the first inputvalue, a control error occurs not only due to a disturbance but also dueto incapability of the correlation model for properly representing anactual correlation between the reference parameter and the first inputvalue, e.g. due to the degraded reliability of the detection results ofthe reference parameter, in other words, due to deviation of thecorrelation model from the actual correlation therebetween, and an errorparameter is calculated so as to represent the control error. In thiscase, as described above, it takes time to compensate for the controlerror represented by the error parameter if the compensation is to becarried out using the second input value parameter calculated with afeedback control algorithm.

In contrast, with the configuration of this control apparatus, theinfluence degree parameter indicative of the degree of influence of thereference parameter on the error parameter is calculated using theinfluence degree model representative of the correlation between theinfluence degree parameter and the reference parameter, and thecorrected error parameter is calculated by correcting the errorparameter by the influence degree parameter, so that, the correctederror parameter is calculated such that it reflects the degree of theinfluence of the reference parameter on the error parameter. Further,the correlation model representative of the correlation between thereference parameter and the first input value is modified based on thecorrected error parameter, and the first input value is calculated usingthe modified correlation model. Therefore, not only when the controlerror is temporarily increased by a disturbance but also when the errorparameter, i.e. the control error is temporarily increased e.g. due tothe degradation of reliability of the detection results of the firstreference parameter, the control error can be properly compensated forjust enough by the first input value.

If the first input value is calculated using the correlation modelmodified according to the error parameter without using the influencedegree parameter in this aspect of the present invention, when thecalculated value of the error parameter is generated mainly by theabove-described deviation of the correlation model, that is, when thedegree of the influence of the reference parameter on the errorparameter is large, the error parameter, i.e. the control error can beproperly compensated for by the first input value calculated as above.However, when the degree of the influence of the reference parameter onthe error parameter is small, i.e. when the error parameter is generatedmainly by a disturbance other than the deviation of the correlationmodel and the like, it is impossible to properly compensate for thecontrol error by the first input value, resulting in overcompensation orundercompensation for the control error. However, to overcome thisproblem, by using the above-described influence degree parameter, thecontrol error can be properly compensated for just enough by the firstinput value. In addition, by using an N (N is a natural number notsmaller than 2) dimensional map which is generally used in thefeedforward control method for representing the correlation between thereference parameter and the first input value, and a calculatingequation representing the correlation therebetween, as the correlationmodel, the control error indicated by the error parameter can becompensated for more quickly than in a case where the same iscompensated for by the second input value. This makes it possible tosuppress the control error from increasing.

As described above, even under the condition where the control error istemporarily increased due to the degraded reliability of the detectionresults of the reference parameter, it is possible to compensate for thecontrol error properly and quickly, thereby making it possible to ensurehigh-level accuracy of control (It should be noted that throughout thespecification, the term “correlation model” is not limited to a responsesurface model or a mathematical model but includes all models whichrepresent the correlation between the reference parameter and the firstinput value, such as the N (N is a natural number not smaller than 2)dimensional map and a predetermined calculation algorithm. Similarly,the throughout the specification, the term “influence degree model” isnot limited to a response surface model or a mathematical model butincludes all models which represent the correlation between theinfluence degree parameter and the reference parameter, such as the Ndimensional map and a predetermined calculation algorithm. Further, theterm “detection of a parameter” is not limited to direct detection ofthe parameter by a sensor, but includes calculation or estimationthereof. In addition thereto, the term “calculation of a parameter” isnot limited to calculation or estimation of the same, but includesdirect detection thereof by a sensor).

Preferably, the predetermined feedback control algorithm is an algorithmto which is applied a predetermined first response-specifying controlalgorithm that specifies a convergence rate of a difference between thecontrolled variable and the target value to 0, and the model-modifyingmeans calculates a modification value with an algorithm to which isapplied a predetermined second response-specifying control algorithmthat specifies a convergence rate of the corrected error parameter to 0,and modifies the correlation model by the modification value, wherein inthe predetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.

With the configuration of the preferred embodiment, the second inputvalue is calculated with the algorithm to which is applied thepredetermined first response-specifying control algorithm that specifiesthe convergence rate of the difference between the controlled variableand the target value to 0, and the modification value for modifying thecorrelation model is calculated with the algorithm to which is appliedthe predetermined second response-specifying control algorithm thatspecifies the convergence rate of the corrected error parameter to 0.When the two response-specifying control algorithms are employed, asdescribed above, if the convergence rates of parameters used therein to0 are set to the same value, there is a fear that the tworesponse-specifying control algorithms interfere with each other,causing an oscillating behavior or an unstable state of the controlsystem. In contrast, according to this control apparatus, theconvergence rate of the corrected error parameter to 0 in thepredetermined second response-specifying control algorithm is set suchthat it becomes lower than the convergence rate of the difference to 0in the predetermined first response-specifying control algorithm,whereby the two response-specifying control algorithms are preventedfrom interfering with each other. This makes it possible to prevent thecontrol system from exhibiting an oscillating behavior due to theinterference between the two response-specifying control algorithms,thereby making it possible to ensure the stability of the controlsystem.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, thereference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

With the configuration of the preferred embodiment, the air-fuel ratioof the mixture is controlled by the amount of fuel to be supplied to theengine, and the amount of fuel to be supplied to the engine iscalculated based on the first input value and the second input value. Acorrelation model representative of the correlation between theoperating condition parameter and the first input value is modifiedaccording to the corrected error parameter, and the first input value iscalculated using the modified correlation model. As a result, even whenthe correlation model ceases to properly represent the actualcorrelation between the operating condition parameter and the firstinput value, due to the degraded reliability of the detection results ofthe operating condition parameter, other than a disturbance, and thecontrol error of the air-fuel ratio is liable to temporarily increase,it is possible to compensate for the increased control error just enoughproperly and quickly by the first input value calculated using themodified correlation model, which makes it possible to prevent thecontrol error from increasing. As a result, it is possible to secure ahigh-level control accuracy of the air-fuel ratio control even in atransient state of the engine.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the referenceparameter being one of a limit value of the output of the engine and arotational speed of the engine.

With the configuration of the preferred embodiment, the wheel speed ofthe vehicle is controlled by the output of the engine, and the output ofthe engine is calculated based on the first and second input values. Thecorrelation model representative of the correlation between the firstinput value and the limit value of the output of the engine or therotational speed of the engine is modified according to the correctederror parameter, and the first input value is calculated using themodified correlation model. Thus, even when the correlation modelbecomes incapable of properly representing the actual correlationbetween the first input value and the limit value of the output of theengine or the rotational speed of the engine, due to unpredictablechanges in conditions other than a disturbance, such as aged degradationof the output characteristics of the engine, variations betweenindividual engines, changes in the degree of wear of tires, and changesin the frictional resistance of road surfaces, and hence the controlerror is liable to temporarily increase, it is possible to properly andquickly compensate for the control error just enough, by the first inputvalue calculated using the modified correlation model, thereby making itpossible to suppress the increase in the control error. As a result, itis possible to ensure higher-level control accuracy of the wheel speedthan by a gain schedule correction (or modification) method. In short, ahigher-level traction control can be realized.

To attain the above object, in a second aspect of the present invention,there is a control apparatus for controlling a controlled variable of acontrolled object by a control input, comprising controlledvariable-detecting means for detecting the controlled variable, firstreference parameter-detecting means for detecting a first referenceparameter of the controlled object other than the controlled variable ofthe controlled object, second reference parameter-detecting means fordetecting a second reference parameter of the controlled object otherthan the controlled variable and the first reference parameter of thecontrolled object, target value-setting means for setting a target valueserving as a target to which the controlled variable is controlled, andcontrol input-calculating means for calculating a first input value forfeedforward control of the controlled variable, according to the firstreference parameter and the second reference parameter, using acorrelation model representative of a correlation between the firstreference parameter, the second reference parameter, and the first inputvalue, calculating a second input value for performing feedback controlof the controlled variable such that the controlled variable is causedto converge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein the control input-calculatingmeans comprises error parameter-calculating means for calculating anerror parameter indicative of a control error to be compensated for bythe first input value, based on the controlled variable and the targetvalue; modification value-calculating means for calculating amodification value for modifying the correlation model according to theerror parameter, first influence degree parameter-calculating means forcalculating a first influence degree parameter indicative of a degree ofinfluence of the first reference parameter on the error parameter, usinga first influence degree model representative of a correlation betweenthe first influence degree parameter and the first reference parameter,corrected modification value-calculating means for calculating acorrected modification value by correcting the modification value by thefirst influence degree parameter, model-modifying means for modifyingthe correlation model according to the corrected modification value, andfirst input value-calculating means for calculating the first inputvalue using the modified correlation model.

In the case of this control apparatus which calculates the first inputvalue for feedforward control of the controlled variable according tothe reference parameter, using the correlation model representative ofthe correlation between the first reference parameter, the secondreference parameter and the first input value, a control error occursnot only due to a disturbance but also due to incapability of thecorrelation model for properly representing an actual correlationbetween the first reference parameter, the second reference parameter,and the first input value, e.g. due to the degraded reliability of thedetection results of the first reference parameter, in other words, dueto deviation of the correlation model from the actual correlationtherebetween, and an error parameter is calculated so as to representthe control error. In this case, as described above, it takes time tocompensate for the control error represented by the error parameter ifthe compensation is to be carried out using the second input valueparameter.

In contrast, with the configuration of this control apparatus, themodification value for modifying the correlation model is calculatedaccording to the error parameter, and the first influence degreeparameter indicative of the degree of influence of the first referenceparameter on the error parameter is calculated using the first influencedegree model representative of the correlation between the firstinfluence degree parameter and the first reference parameter. Thecorrected modification value is calculated by correcting themodification value by the first influence degree parameter. Thus, thecorrected modification value is calculated such that it reflects thedegree of influence of the first reference parameter on the errorparameter. Further, the correlation model is modified using thecorrected modification value thus calculated, and the first input valueis calculated using the modified correlation model. Therefore, even whenthe control error indicated by the error parameter is temporarilyincreased due to the degraded reliability of the detection results ofthe first reference parameter, it is possible to properly compensate forthe control error just enough by the first input value calculated asabove.

If the first input value is calculated using the correlation modelmodified by the modification value without using the first influencedegree parameter in this aspect of the present invention, when thecalculated value of the error parameter is generated mainly by theabove-described deviation of the correlation model, that is, when thedegree of the influence of the first reference parameter on the errorparameter is large, the control error indicated by the error parametercan be properly compensated for by the first input value calculated asabove. However, when the degree of the influence of the first referenceparameter on the error parameter is small, i.e. when the error parameteris generated mainly by a disturbance other than the deviation of thecorrelation model and the like, it is impossible to properly compensatefor the control error by the first input value, resulting inovercompensation or undercompensation for the control error. Therefore,by using the above-described first influence degree parameter, thecontrol error can be properly compensated for just enough by the firstinput value.

In addition, by using an M (M is a natural number not smaller than 3)dimensional map which is generally used in the feedforward controlmethod for representing the correlation between the first referenceparameter, the second reference parameter, and the first input value,and a calculating equation representing the correlation therebetween,for the correlation model, the control error can be compensated for morequickly than in a case where the error parameter is compensated for bythe second input value. As described above, even under a condition wherethe control error is temporarily increased e.g. due to the degradedreliability of the detection results of the first reference parameter,it is possible to compensate for the control error properly and quickly,thereby making it possible to ensure high-level accuracy of control (Itshould be noted that throughout the specification, the term “firstinfluence degree model” is not limited to a response surface model or amathematical model but includes all models which represent thecorrelation between the first influence degree parameter and thereference parameter, such as the N dimensional map and a predeterminedcalculation algorithm).

Preferably, the control apparatus further comprises second influencedegree parameter-calculating means for calculating a second influencedegree parameter indicative of a degree of influence of the secondreference parameter on the error parameter, using a second influencedegree model representative of a correlation between the secondinfluence degree parameter and the second reference parameter, andcorrected error parameter-calculating means for calculating a correctederror parameter by correcting the error parameter by the secondinfluence degree parameter; wherein the modification value-calculatingmeans calculates the modification value according to the corrected errorparameter.

With the configuration of the preferred embodiment, the second influencedegree parameter indicative of the degree of influence of the secondreference parameter on the error parameter is calculated using thesecond influence degree model representative of the correlation betweenthe second influence degree parameter and the second referenceparameter. The corrected error parameter is calculated by correcting theerror parameter by the second influence degree parameter, and themodification value is calculated according to the corrected errorparameter. Therefore, the modification value is calculated such that itreflects the degree of influence of the second reference parameter onthe error parameter. Further, the correlation model is modified usingthe corrected modification value obtained by correcting the modificationvalue, and the first input value is calculated using the modifiedcorrelation model, so that even under a condition where the controlerror indicated by the error parameter is temporarily increased by thethus calculated first input value, the control error can be properlycompensated for just enough. In addition, by using the M (M is a naturalnumber not smaller than 3) dimensional map which is generally used inthe feedforward control method for representing the correlation betweenthe first reference parameter, the second reference parameter, and thefirst input value, and a calculating equation representing thecorrelation therebetween, for the correlation model, the control errorindicated by the error parameter can be compensated for more quicklythan in a case where the error parameter is compensated for by thesecond input value. As described above, even under a condition where thecontrol error is temporarily increased not by a disturbance but e.g. bythe degraded reliability of the detection results of the secondreference parameter, it is possible to compensate for the control errorproperly and quickly, thereby making it possible to ensure ahigher-level accuracy of control (It should be noted that throughout thespecification, “second influence degree model” is not limited to aresponse surface model or a mathematical model but includes all modelswhich represent the correlation between the reference parameter and thefirst input value, such as the N dimensional map and a predeterminedcalculation algorithm.

More preferably, the predetermined feedback control algorithm is analgorithm to which is applied a predetermined first response-specifyingcontrol algorithm for specifying a convergence rate of a differencebetween the controlled variable and the target value to 0, and themodification value-calculating means calculates the modification valuewith an algorithm to which is applied a predetermined secondresponse-specifying control algorithm that specifies a convergence rateof the corrected error parameter to 0, wherein in the predeterminedsecond response-specifying control algorithm, the convergence rate ofthe corrected error parameter to 0 is set such that it becomes lowerthan the convergence rate of the difference to 0 in the predeterminedfirst response-specifying control algorithm.

With the configurations of these preferred embodiments, it is possibleto obtain the same advantageous effects as provided by the correspondingpreferred embodiment of the first aspect of the present invention.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, the secondreference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

With the configuration of the preferred embodiment, the air-fuel ratioof the mixture is controlled by the amount of fuel to be supplied to theengine, and the amount of fuel to be supplied to the engine iscalculated based on the first input value and the second input value. Acorrelation model representative of the correlation between the firstreference parameter, the operating condition parameter, and the firstinput value is modified according to the corrected error parameter, andthe first input value is calculated using the modified correlationmodel. As a result, even when the correlation model ceases to properlyrepresent the actual correlation between the first reference parameter,the operating condition parameter, and the first input value, due to thedegraded reliability of the detection results of the operating conditionparameter and the like, and the control error of the air-fuel ratio isliable to temporarily increase, it is possible to compensate for theincreased control error just enough properly and quickly by the firstinput value calculated using the modified correlation model, which makesit possible to prevent the control error from increasing. As a result,it is possible to secure a high-level control accuracy of the air-fuelratio control even in a transient state of the engine.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the secondreference parameter being one of a limit value of the output of theengine and a rotational speed of the engine.

With the configuration of the preferred embodiment, the wheel speed ofthe vehicle is controlled by the output of the engine, and the output ofthe engine is calculated based on the first and second input values. Thecorrelation model representative of the correlation between the firstreference parameter, the limit value of the output of the engine or therotational speed of the engine, and the first input value is modifiedaccording to the corrected error parameter, and the first input value iscalculated using the modified correlation model. Thus, even when thecorrelation model becomes incapable of properly representing the actualcorrelation between the first reference parameter, the limit value ofthe output of the engine or the rotational speed of the engine, and thefirst input value due to unpredictable changes in conditions other thana disturbance, such as aged degradation of the output characteristics ofthe engine, variations between individual engines, changes in the degreeof wear of tires, and changes in the frictional resistance of roadsurfaces, and hence the control error is liable to temporarily increase,it is possible to properly and quickly compensate for the control errorjust enough, by the first input value calculated using the modifiedcorrelation model, thereby making it possible to suppress the increasein the control error. As a result, it is possible to ensure higher-levelcontrol accuracy of the wheel speed than by a gain schedule correction(or modification) method. In short, a higher-level traction control canbe realized.

To attain the object, in a third aspect of the present invention, thereis provided a method of controlling a controlled variable of acontrolled object by a control input, comprising a controlledvariable-detecting step of detecting the controlled variable, areference parameter-detecting step of detecting a reference parameter ofthe controlled object other than the controlled variable of thecontrolled object, a target value-setting step of setting a target valueserving as a target to which the controlled variable is controlled, anda control input-calculating step of calculating a first input value forfeedforward control of the controlled variable, according to thereference parameter, using a correlation model representative of acorrelation between the reference parameter and the first input value,calculating a second input value for performing feedback control of thecontrolled variable such that the controlled variable is caused toconverge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein the control input-calculatingstep comprises an error parameter-calculating step of calculating anerror parameter indicative of a control error to be compensated for bythe first input value, based on the controlled variable and the targetvalue, an influence degree parameter-calculating step of calculating aninfluence degree parameter indicative of a degree of influence of thereference parameter on the error parameter by using an influence degreemodel representative of a correlation between the influence degreeparameter and the reference parameter, a corrected errorparameter-calculating step of calculating a corrected error parameter bycorrecting the error parameter by the influence degree parameter, amodel-modifying step of modifying the correlation model according to thecorrected error parameter, and a first input value-calculating step ofcalculating the first input value using the modified correlation model.

With the configuration of the third aspect of the present invention, itis possible to obtain the same advantageous effects as provided by thefirst aspect of the present invention.

Preferably, the predetermined feedback control algorithm is an algorithmto which is applied a predetermined first response-specifying controlalgorithm that specifies a convergence rate of a difference between thecontrolled variable and the target value to 0, the model-modifying stepincluding calculating a modification value with an algorithm to which isapplied a predetermined second response-specifying control algorithmthat specifies a convergence rate of the corrected error parameter to 0,and modifying the correlation model by the modification value, and inthe predetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, thereference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the referenceparameter being one of a limit value of the output of the engine and arotational speed of the engine.

With the configurations of these preferred embodiments, it is possibleto obtain the same advantageous effects as provided by the preferredembodiments of the first aspect of the present invention.

To attain the object, in a fourth aspect of the present invention, thereis provided a method of controlling a controlled variable of acontrolled object by a control input, comprising a controlledvariable-detecting step of detecting the controlled variable, a firstreference parameter-detecting step of detecting a first referenceparameter of the controlled object other than the controlled variable ofthe controlled object, a second reference parameter-detecting step ofdetecting a second reference parameter of the controlled object otherthan the controlled variable and the first reference parameter of thecontrolled object, a target value-setting step of setting a target valueserving as a target to which the controlled variable is controlled, anda control input-calculating step of calculating a first input value forfeedforward control of the controlled variable, according to the firstreference parameter and the second reference parameter, using acorrelation model representative of a correlation between the firstreference parameter, the second reference parameter, and the first inputvalue, calculating a second input value for performing feedback controlof the controlled variable such that the controlled variable is causedto converge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein the control input-calculatingstep comprises an error parameter-calculating step of calculating anerror parameter indicative of a control error to be compensated for bythe first input value, based on the controlled variable and the targetvalue, a modification value-calculating step of calculating amodification value for modifying the correlation model according to theerror parameter, a first influence degree parameter-calculating step ofcalculating a first influence degree parameter indicative of a degree ofinfluence of the first reference parameter on the error parameter, usinga first influence degree model representative of a correlation betweenthe first influence degree parameter and the first reference parameter,a corrected modification value-calculating step of calculating acorrected modification value by correcting the modification value by thefirst influence degree parameter, a model-modifying step of modifyingthe correlation model according to the corrected modification value, anda first input value-calculating step of calculating the first inputvalue using the modified correlation model.

With the configuration of the fourth aspect of the present invention, itis possible to obtain the same advantageous effects as provided by thesecond aspect of the present invention.

Preferably, the method further comprises a second influence degreeparameter-calculating step of calculating a second influence degreeparameter indicative of a degree of influence of the second referenceparameter on the error parameter, using a second influence degree modelrepresentative of a correlation between the second influence degreeparameter and the second reference parameter, and a corrected errorparameter-calculating step of calculating a corrected error parameter bycorrecting the error parameter by the second influence degree parameter,wherein the modification value-calculating step includes calculating themodification value according to the corrected error parameter.

More preferably, the predetermined feedback control algorithm is analgorithm to which is applied a predetermined first response-specifyingcontrol algorithm for specifying a convergence rate of a differencebetween the controlled variable and the target value to 0, themodification value-calculating step including calculating themodification value with an algorithm to which is applied a predeterminedsecond response-specifying control algorithm that specifies aconvergence rate of the corrected error parameter to 0, and in thepredetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, the secondreference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the secondreference parameter being one of a limit value of the output of theengine and a rotational speed of the engine.

With the configurations of these preferred embodiments, it is possibleto obtain the same advantageous effects as provided by the preferredembodiments of the second aspect of the present invention.

To attain the object, in a fifth aspect of the present invention, thereis provided an engine control unit including a control program forcausing a computer to execute a method of controlling a controlledvariable of a controlled object by a control input, wherein the controlprogram causes the computer to detect the controlled variable; detect areference parameter of the controlled object other than the controlledvariable of the controlled object; set a target value serving as atarget to which the controlled variable is controlled; and calculate afirst input value for feedforward control of the controlled variable,according to the reference parameter, using a correlation modelrepresentative of a correlation between the reference parameter and thefirst input value, calculate a second input value for performingfeedback control of the controlled variable such that the controlledvariable is caused to converge to the target value, with a predeterminedfeedback control algorithm, and calculate the control input based on thefirst input value and the second input value, wherein when causing thecomputer to calculate the control input, the control program causes thecomputer to calculate an error parameter indicative of a control errorto be compensated for by the first input value, based on the controlledvariable and the target value; calculate an influence degree parameterindicative of a degree of influence of the reference parameter on theerror parameter by using an influence degree model representative of acorrelation between the influence degree parameter and the referenceparameter; calculate a corrected error parameter by correcting the errorparameter by the influence degree parameter; modify the correlationmodel according to the corrected error parameter; and calculate thefirst input value using the modified correlation model.

With the configuration of the fifth aspect of the present invention, itis possible to obtain the same advantageous effects as provided by thefirst aspect of the present invention.

Preferably, the predetermined feedback control algorithm is an algorithmto which is applied a predetermined first response-specifying controlalgorithm that specifies a convergence rate of a difference between thecontrolled variable and the target value to 0, the control programcausing, when causing the computer to modify the correlation model, thecomputer to calculate a modification value with an algorithm to which isapplied a predetermined second response-specifying control algorithmthat specifies a convergence rate of the corrected error parameter to 0,and modify the correlation model by the modification value, and in thepredetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, thereference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the referenceparameter being one of a limit value of the output of the engine and arotational speed of the engine.

With the configurations of these preferred embodiments, it is possibleto obtain the same advantageous effects as provided by the preferredembodiments of the first aspect of the present invention.

To attain the object, in a sixth aspect of the present invention, thereis provided an engine control unit including a control program forcausing a computer to execute a method of controlling a controlledvariable of a controlled object by a control input, wherein the controlprogram causes the computer to detect the controlled variable; detect afirst reference parameter of the controlled object other than thecontrolled variable of the controlled object; detect a second referenceparameter of the controlled object other than the controlled variableand the first reference parameter of the controlled object; set a targetvalue serving as a target to which the controlled variable iscontrolled; and calculate a first input value for feedforward control ofthe controlled variable, according to the first reference parameter andthe second reference parameter, using a correlation model representativeof a correlation between the first reference parameter, the secondreference parameter, and the first input value, calculating a secondinput value for performing feedback control of the controlled variablesuch that the controlled variable is caused to converge to the targetvalue, with a predetermined feedback control algorithm, and calculatingthe control input based on the first input value and the second inputvalue, wherein when causing the computer to calculate the control input,the control program causes the computer to calculate an error parameterindicative of a control error to be compensated for by the first inputvalue, based on the controlled variable and the target value; calculatea modification value for modifying the correlation model according tothe error parameter; calculate a first influence degree parameterindicative of a degree of influence of the first reference parameter onthe error parameter, using a first influence degree model representativeof a correlation between the first influence degree parameter and thefirst reference parameter; calculate a corrected modification value bycorrecting the modification value by the first influence degreeparameter; modify the correlation model according to the correctedmodification value; and calculate the first input value using themodified correlation model.

With the configuration of the sixth aspect of the present invention, itis possible to obtain the same advantageous effects as provided by thesecond aspect of the present invention.

Preferably, the control program further causes the computer to calculatea second influence degree parameter indicative of a degree of influenceof the second reference parameter on the error parameter, using a secondinfluence degree model representative of a correlation between thesecond influence degree parameter and the second reference parameter;and calculate a corrected error parameter by correcting the errorparameter by the second influence degree parameter, and when causing thecomputer to calculate the modification value, the control program causesthe computer to calculate the modification value according to thecorrected error parameter.

More preferably, the predetermined feedback control algorithm is analgorithm to which is applied a predetermined first response-specifyingcontrol algorithm for specifying a convergence rate of a differencebetween the controlled variable and the target value to 0, the controlprogram causing, when causing the computer to calculate the modificationvalue, the computer to calculate the modification value with analgorithm to which is applied a predetermined second response-specifyingcontrol algorithm that specifies a convergence rate of the correctederror parameter to 0, and in the predetermined secondresponse-specifying control algorithm, the convergence rate of thecorrected error parameter to 0 is set such that it becomes lower thanthe convergence rate of the difference to 0 in the predetermined firstresponse-specifying control algorithm.

Preferably, the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, the secondreference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.

Preferably, the controlled object is a vehicle using the engine as adrive source thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the secondreference parameter being one of a limit value of the output of theengine and a rotational speed of the engine.

With the configurations of these preferred embodiments, it is possibleto obtain the same advantageous effects as provided by the preferredembodiments of the second aspect of the present invention.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an internal combustion engine to whichis applied a control apparatus according to a first embodiment of thepresent invention;

FIG. 2 is a schematic block diagram of the control apparatus;

FIG. 3 is a schematic cross-sectional view of a variable intakevalve-actuating mechanism and an exhaust valve-actuating mechanism ofthe engine;

FIG. 4 is a schematic cross-sectional view of a variable valve liftmechanism of the variable intake valve-actuating mechanism;

FIG. 5A is a diagram showing a lift actuator in a state in which a shortarm thereof is in a maximum lift position;

FIG. 5B is a diagram showing the lift actuator in a state in which theshort arm thereof is in a zero position;

FIG. 6A is a diagram showing an intake valve placed in an open statewhen a lower link of the variable valve lift mechanism is in a maximumlift position;

FIG. 6B is a diagram showing the intake valve placed in a stopped statewhen the lower link of the variable valve lift mechanism is in the zerolift position;

FIG. 7 is a diagram showing a valve lift curve (solid line) of theintake valve obtained when the lower link of the variable valve liftmechanism is in the maximum lift position, and a valve lift curve(two-dot chain line) of the intake valve obtained when the lower link ofthe variable valve lift mechanism is in the zero lift position;

FIG. 8 is a schematic diagram of a variable cam phase mechanism;

FIG. 9 is a diagram showing a valve lift curve (solid line) obtainedwhen a cam phase is set to a most retarded value by the variable camphase mechanism, and a valve lift curve (two-dot chain line) obtainedwhen the cam phase is set to a most advanced value by the variable camphase mechanism;

FIG. 10 is a schematic block diagram of an air-fuel ratio controller;

FIG. 11 is a diagram showing an example of a map for use in calculatinga basic estimated intake air amount;

FIG. 12 is a diagram showing an example of a map for use in calculatinga correction coefficient;

FIG. 13 is a diagram showing an example of a map for use in calculatinga transition coefficient;

FIG. 14 is a diagram showing an example of a map for use in calculatinga target air-fuel ratio;

FIG. 15 is a diagram showing a state in which a lift error is caused byan offset of a calculated value of the valve lift with respect to theactual value thereof;

FIG. 16 is a diagram showing a state in which a lift error is caused bya change in dynamic characteristics of the variable valve liftmechanism;

FIG. 17 is a diagram showing the relationship between an amount ofchange in the basic estimated intake air amount and an amount of changein the valve lift;

FIG. 18 is a schematic block diagram of a lift correctionvalue-calculating section;

FIG. 19 is a diagram showing an example of a map for use in calculatinga basic error weight;

FIG. 20 is a diagram showing an example of a map for use in calculatingan error weight correction coefficient;

FIG. 21 is a diagram showing an example of a map for use in calculatinga basic sensitivity;

FIG. 22 is a diagram showing an example of a map for use in calculatinga sensitivity correction coefficient;

FIG. 23 is a flowchart of a control process executed at a controlperiod;

FIG. 24 is a flowchart of an air-fuel ratio control process;

FIG. 25 is a flowchart of a process for calculating a basic fuelinjection amount;

FIG. 26 is a flowchart of a control process executed at a controlperiod;

FIG. 27 is a flowchart of a process for calculating a corrected valvelift;

FIG. 28 is a flowchart of a variable mechanism control process;

FIG. 29 is a diagram showing an example of a map for use in calculatinga target valve lift during the start of the engine;

FIG. 30 is a diagram showing an example of a map for use in calculatinga target cam phase during the start of the engine;

FIG. 31 is a diagram showing an example of a map for use in calculatingthe target valve lift during catalyst warmup control;

FIG. 32 is a diagram showing an example of a map for use in calculatingthe target cam phase during catalyst warmup control;

FIG. 33 is a diagram showing an example of a map for use in calculatingthe target valve lift during normal operation of the engine;

FIG. 34 is a diagram showing an example of a map for use in calculatingthe target cam phase during normal operation of the engine;

FIG. 35 is a timing diagram showing an example of a result of controlexecuted by the control apparatus according to the first embodiment;

FIG. 36 is a timing diagram showing an example of a result of air-fuelratio control executed by the control apparatus according to the firstembodiment;

FIG. 37 is a timing diagram showing a comparative example of a result ofair-fuel ratio control, obtained when a lift correction value is held at0;

FIG. 38 is a schematic block diagram of a control apparatus according toa second embodiment of the present invention;

FIG. 39 is a schematic block diagram of a traction controller;

FIG. 40 is a diagram showing an example of a map for use in calculatinga maximum torque and a minimum torque;

FIG. 41 is a diagram showing an example of a map for use in calculatinga normalization demand driving force;

FIG. 42 is a schematic block diagram of a torque correctionvalue-calculating section;

FIG. 43 is a diagram showing an example of a map for use in calculatingan error weight;

FIG. 44 is a diagram showing an example of a map for use in calculatinga torque correction sensitivity;

FIG. 45 is a timing diagram showing an example of results of tractioncontrol executed by the control apparatus according to the secondembodiment; and

FIG. 46 is a timing diagram showing an example of results of thetraction control, obtained when a torque correction value=1 holds forcomparison with the FIG. 45 example.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereafter, a control apparatus according to a first embodiment of thepresent invention will be described with reference to the drawings. Asshown in FIG. 2, the control apparatus 1 includes an ECU 2. As describedhereinafter, the ECU 2 carries out control processes, such as anair-fuel ratio control process, depending on operating conditions of aninternal combustion engine, which is a controlled object.

Referring to FIGS. 1 and 3, an internal combustion engine (hereinaftersimply referred to as “the engine”) 3 is an in-line four-cylindergasoline engine having four pairs of cylinders 3 a and pistons 3 b (onlyone pair of which is shown), and installed on a vehicle with anautomatic transmission, not shown. The engine 3 includes an intake valve4 and an exhaust valve 7 provided for each cylinder 3 a, for opening andclosing an intake port and an exhaust port thereof, respectively, anintake camshaft 5 and intake cams 6 for actuating the intake valves 4, avariable intake valve-actuating mechanism 40 that actuates the intakevalves 4 to open and close the same, an exhaust camshaft 8 and exhaustcams 9 for actuating the exhaust valves 7, an exhaust valve-actuatingmechanism 30 that actuates the exhaust valves 7 to open and close thesame, fuel injection valves 10, spark plugs 11 (see FIG. 2), and soforth.

The intake valve 4 has a stem 4 a thereof slidably fitted in a guide 4b. The guide 4 b is rigidly fixed to a cylinder head 3 c. Further, asshown in FIG. 4, the intake valve 4 includes upper and lower springsheets 4 c and 4 d, and a valve spring 4 e disposed therebetween, andthe stem 4 a is urged by the valve spring 4 e in the valve-closingdirection.

Further, the intake camshaft 5 and the exhaust camshaft 8 are rotatablymounted through the cylinder head 3 c via holders, not shown. The intakecamshaft 5 has an intake sprocket (not shown) coaxially and rotatablyfitted on one end thereof. The intake sprocket is connected to acrankshaft 3 d via a timing chain, not shown, and connected to theintake camshaft via a variable cam phase mechanism 70, describedhereinafter. With the above arrangement, the intake camshaft 5 performsone rotation per two rotations of the crankshaft 3 d. Further, theintake cam 6 is provided on the intake camshaft 5 for each cylinder 3 asuch that the intake cam 6 rotates in unison with the intake camshaft 5.

Furthermore, the variable intake valve-actuating mechanism 40 isprovided for actuating the intake valve 4 of each cylinder 3 a so as toopen and close the same, in accordance with rotation of the intakecamshaft 5, and continuously changing the lift and the valve timing ofthe intake valve 4, which will be described in detail hereinafter. Itshould be noted that in the present embodiment, “the lift of the intakevalve 4” (hereinafter referred to as “the valve lift”). represents themaximum lift of the intake valve 4.

On the other hand, the exhaust valve 7 has a stem 7 a thereof slidablyfitted in a guide 7 b. The guide 7 b is rigidly fixed to the cylinderhead 3 c. Further, the exhaust valve 7 includes upper and lower springsheets 7 c and 7 d, and a valve spring 7 e disposed therebetween, andthe stem 7 a is urged by the valve spring 7 e in the valve-closingdirection.

Further, the exhaust camshaft 8 has an exhaust sprocket (not shown)integrally formed therewith, and is connected to the crankshaft 3 d bythe exhaust sprocket and the timing chain, not shown, whereby theexhaust camshaft 8 performs one rotation per two rotations of thecrankshaft 3 d. Further, the exhaust cam 9 is provided on the exhaustcamshaft 8 for each cylinder 3 a such that the exhaust cam 9 rotates inunison with the exhaust camshaft 8.

Further, the exhaust valve-actuating mechanism 30 includes rocker arms31. Each rocker arm 31 is pivotally moved in accordance with rotation ofthe associated exhaust cam 9 to thereby actuate the exhaust valve 7 foropening and closing the same against the urging force of the valvespring 7 e.

On the other hand, the fuel injection valve 10 is provided for eachcylinder 3 a, and mounted through the cylinder head 3 c in a tiltedstate such that fuel is directly injected into a combustion chamber.That is, the engine 3 is configured as a direct injection engine.Further, the fuel injection valve 10 is electrically connected to theECU 2 and the valve-opening time period and the valve-opening timingthereof are controlled by the ECU 2, whereby the fuel injection amountis controlled.

The spark plug 11 as well is provided for each cylinder 3 a, and mountedthrough the cylinder head 3 c. The spark plug 11 is electricallyconnected to the ECU 2, and a state of spark discharge is controlled bythe ECU 2 such that a mixture in the combustion chamber is burned intiming corresponding to ignition timing.

On the other hand, the engine 3 is provided with a crank angle sensor 20and an engine coolant temperature sensor 21. The crank angle sensor 20is comprised of a magnet rotor and an MRE (magnetic resistance element)pickup, and delivers a CRK signal and a TDC signal, which are both pulsesignals, to the ECU 2 in accordance with rotation of the crankshaft 3 d.

Each pulse of the CRK signal is generated whenever the crankshaft 3 drotates through a predetermined angle (e.g. 10). The ECU 2 calculatesthe rotational speed NE of the engine 3 (hereinafter referred to as “theengine speed NE”) based on the CRK signal. The TDC signal indicates thatthe piston 3 b has come to a predetermined crank angle positionimmediately before the TDC position at the start of the intake stroke,on a cylinder-by-cylinder basis, and each pulse thereof is generatedwhenever the crankshaft 3 d rotates through a predetermined crank angle.It should be noted that in the present embodiment, the crank anglesensor 20 corresponds to first reference parameter-detecting means, andthe engine speed NE corresponds to a first reference parameter.

The engine coolant temperature sensor 21 is implemented e.g. by athermistor, and detects an engine coolant temperature TW to deliver asignal indicative of the sensed engine coolant temperature TW to the ECU2. The engine coolant temperature TW is the temperature of an enginecoolant circulating through a cylinder block 3 h of the engine 3.

Further, the engine 3 has an intake pipe 12 from which a throttle valvemechanism is omitted, and an intake passage 12 a having a large diameteris formed through the intake pipe 12, whereby the engine 3 is configuredsuch that flow resistance is smaller than in an ordinary engine. Theintake pipe 12 is provided with an air flow sensor 22 and an intake airtemperature sensor 23 (see FIG. 2).

The air flow sensor 22 is implemented by a hot-wire air flow meter, anddetects the flow rate Gin of air (hereinafter referred to as “the airflow rate Gin”) flowing through the intake passage 12 a to deliver asignal indicative of the sensed air flow rate Gin to the ECU 2. Itshould be noted that the air flow rate Gin is indicated in units ofg/sec. Further, the intake air temperature sensor 23 detects thetemperature TA of intake air (hereinafter referred to as “the intake airtemperature TA”) flowing through the intake passage 12 a, and delivers asignal indicative of the sensed intake air temperature TA to the ECU 2.

Further, a LAF sensor 24 and a catalytic device 14 are provided in theexhaust pipe 13 at respective locations in the mentioned order from theupstream side. The LAF sensor 24 is comprised of a zirconia layer andplatinum electrodes, and linearly detects the concentration of oxygen inexhaust gases flowing through an exhaust passage 13 a of the exhaustpipe 13, in a broad air-fuel ratio range from a rich region richer thana stoichiometric air-fuel ratio to a very lean region, and delivers asignal indicative of the sensed oxygen concentration to the ECU 2.

The ECU 2 calculates an actual air-fuel ratio KACT indicative of theair-fuel ratio in exhaust gases, based on the value of the signal fromthe LAF sensor 24. In this case, the actual air-fuel ratio KACT iscalculated as an equivalent ratio. It should be noted that in thepresent embodiment, the LAF sensor 24 corresponds to controlledvariable-detecting means, and the actual air-fuel ratio KACT correspondsto a controlled variable.

Next, a description will be given of the aforementioned variable intakevalve-actuating mechanism 40. As shown in FIG. 4, the variable intakevalve-actuating mechanism 40 is comprised of the intake camshaft 5, theintake cams 6, a variable valve lift mechanism 50, and the variable camphase mechanism 70.

The variable valve lift mechanism 50 actuates the intake valves 4 toopen and close the same, in accordance with rotation of the intakecamshaft 5, and continuously changes the valve lift Liftin between apredetermined maximum value Liftinmax and 0. The variable valve liftmechanism 50 is comprised of rocker arm mechanisms 51 of a four jointlink type, provided for the respective cylinders 3 a, and a liftactuator 60 (see FIGS. 5A and 5B) simultaneously actuating these rockerarm mechanisms 51. It should be noted that in the present embodiment,the variable valve lift mechanism 50 corresponds to a variable intakemechanism.

Each rocker arm mechanism 51 is comprised of a rocker arm 52, and upperand lower links 53 and 54. The upper link 53 has one end pivotallymounted to an upper end of the rocker arm 52 by an upper pin 55, and theother end pivotally mounted to a rocker arm shaft 56. The rocker armshaft 56 is mounted through the cylinder head 3 c via holders, notshown.

Further, a roller 57 is pivotally disposed on the upper pin 55 of therocker arm 52. The roller 57 is in contact with a cam surface of theintake cam 6. As the intake cam 6 rotates, the roller 57 rolls on theintake cam 6 while being guided by the cam surface of the intake cam 6.As a result, the rocker arm 52 is vertically driven, and the upper link53 is pivotally moved about the rocker arm shaft 56.

Furthermore, an adjusting bolt 52 a is mounted to an end of the rockerarm 52 toward the intake valve 4. When the rocker arm 52 is verticallymoved in accordance with rotation of the intake cam 6, the adjustingbolt 52 a vertically drives the stem 4 a to open and close the intakevalve 4, against the urging force of the valve spring 4 e.

Further, the lower link 54 has one end pivotally mounted to a lower endof the rocker arm 52 by a lower pin 58, and the other end of the lowerlink 54 has a connection shaft 59 pivotally mounted thereto. The lowerlink 54 is connected to a short arm 65, described hereinafter, of thelift actuator 60 by the connection shaft 59.

On the other hand, as shown in FIGS. 5A and 5B, the lift actuator 60 iscomprised of a motor 61, a nut 62, a link 63, a long arm 64, and theshort arm 65. The motor 61 is connected to the ECU 2, and disposedoutside a head cover 3 g of the engine 3. The rotary shaft of the motor61 is a screw shaft 61 a formed with a male screw and the nut 62 isscrewed onto the screw shaft 61 a. The nut 62 is connected to the longarm 64 by the link 63. The link 63 has one end pivotally mounted to thenut 62 by a pin 63 a, and the other end pivotally mounted to one end ofthe long arm 64 by a pin 63 b.

Further, the other end of the long arm 64 is attached to one end of theshort arm 65 by a pivot shaft 66. The pivot shaft 66 is circular incross section, and extends through the head cover 3 g of the engine 3such that it is pivotally supported by the head cover 3 g. The long arm64 and the short arm 65 are pivotally moved in unison with the pivotshaft 66 in accordance with pivotal motion of the pivot shaft 66.

Furthermore, the aforementioned connection shaft 59 rotatably extendsthrough the other end of the short arm 65, whereby the short arm 65 isconnected to the lower link 54 by the connection shaft 59.

Next, a description will be given of the operation of the variable valvelift mechanism 50 configured as above. In the variable valve liftmechanism 50, when a lift control input U_Liftin, described hereinafter,is input from the ECU 2 to the lift actuator 60, the screw shaft 61 arotates, and the nut 62 is moved in accordance with the rotation of thescrew shaft 61 a, whereby the long arm 64 and the short arm 65 arepivotally moved about the pivot shaft 66, and in accordance with thepivotal motion of the short arm 65, the lower link 54 of the rocker armmechanism 51 is pivotally moved about the lower pin 58. That is, thelower link 54 is driven by the lift actuator 60.

During the above process, under the control of the ECU 2, the range ofpivotal motion of the short arm 65 is restricted between the maximumlift position shown in FIG. 5A and the zero lift position shown in FIG.5B, whereby the range of pivotal motion of the lower link 54 is alsorestricted between the maximum lift position indicated by the solid linein FIG. 4 and the zero lift position indicated by the two-dot chain linein FIG. 4.

The four joint link formed by the rocker arm shaft 56, the upper andlower pins 55 and 58, and the connection shaft 59 is configured suchthat when the lower link 54 is in the maximum lift position, thedistance between the center of the upper pin 55 and the center of thelower pin 58 becomes longer than the distance between the center of therocker arm shaft 56 and the center of the connection shaft 59, wherebyas shown in FIG. 6A, when the intake cam 6 rotates, the amount ofmovement of the adjusting bolt 52 a becomes larger than the amount ofmovement of a contact point where the intake cam 6 and the roller 57 arein contact with each other.

On the other hand, the four joint link is configured such that when thelower link 54 is in the zero lift position, the distance between thecenter of the upper pin 55 and the center of the lower pin 58 becomesshorter than the distance between the center of the rocker arm shaft 56and the center of the connection shaft 59, whereby as shown in FIG. 6B,the adjusting bolt 52 a is placed in a state substantially immovablewhen the intake cam 6 rotates.

For the above reason, during rotation of the intake cam 6, when thelower link 54 is in the maximum lift position, the intake valve 4 isopened according to a valve lift curve indicated by a solid line in FIG.7, and the valve lift Liftin takes its maximum value Liftinmax. On theother hand, when the lower link 54 is in the zero lift position, asindicated by a two-dot chain line in FIG. 7, the intake valve 4 is heldin the closed state, and the valve lift Liftin is held at 0.

Therefore, in the variable valve lift mechanism 50, the lower link 54 ispivotally moved by the lift actuator 60 between the maximum liftposition and the zero lift position, whereby it is possible tocontinuously change the valve lift Liftin between the maximum valueLiftinmax and 0.

It should be noted that the variable valve lift mechanism 50 includes alock mechanism, not shown, and the lock mechanism locks the operation ofthe variable valve lift mechanism 50 when the lift control inputU_Liftin is set to a failure-time value U_Liftin_fs, as describedhereinafter, or when the lift control input U_Liftin is not input fromthe ECU 2 to the lift actuator 60 e.g. due to a disconnection. That is,the variable valve lift mechanism 50 is inhibited from changing thevalve lift Liftin, whereby the valve lift Liftin is held at apredetermined locked value. It should be noted that when a cam phaseCain is held at a locked value, described hereinafter, the predeterminedlocked value is set to such a value as will make it possible to ensure apredetermined failure-time value Gcyl_fs of the intake air amount,described hereinafter. The predetermined failure-time value Gcyl_fs isset to a value which is capable of suitably carrying out idling orstarting of the engine 3 during stoppage of the vehicle, and capable ofmaintaining a low-speed traveling state of the vehicle during travel ofthe vehicle.

The engine 3 is provided with a pivot angle sensor 25 (see FIG. 2). Thepivot angle sensor 25 detects a pivot angle of the pivot shaft 66, i.e.the short arm 65, and delivers a signal indicative of the detected pivotangle of the short arm 65 to the ECU 2. The ECU 2 calculates the valvelift Liftin based on the detection signal from the pivot angle sensor25. It should be noted that in the present embodiment, the pivot anglesensor 25 corresponds to reference parameter-detecting means and secondreference parameter-detecting means, and the valve lift Liftincorresponds to a reference parameter, a second reference parameter, andan operating state parameter.

Next, a description will be given of the aforementioned variable camphase mechanism 70. The variable cam phase mechanism 70 is provided forcontinuously advancing or retarding the relative phase Cain of theintake camshaft 5 with respect to the crankshaft 3 d (hereinafterreferred to as “the cam phase Cain”), and mounted on an intakesprocket-side end of the intake camshaft 5. As shown in FIG. 8, thevariable cam phase mechanism 70 includes a housing 71, a three-bladedvane 72, an oil pressure pump 73, and a solenoid valve mechanism 74.

The housing 71 is integrally formed with the intake sprocket on theintake camshaft 5 d, and divided by three partition walls 71 a formed atequal intervals. The vane 72 is coaxially mounted on the end of theintake camshaft 5 where the intake sprocket is mounted, such that theblades of the vane 72 radially extends outward from the intake camshaft5, and are rotatably housed in the housing 71. Further, the housing 71has three advance chambers 75 and three retard chambers 76 each formedbetween one of the partition walls 71 a and one of the three blades ofthe vane 72.

The oil pressure pump 73 is a mechanically-driven type which isconnected to the crankshaft 3 d. As the crankshaft 3 d rotates, the oilpressure pump 73 draws lubricating oil stored in an oil pan 3 e of theengine 3 via a lower part of an oil passage 77 c, for pressurization,and supplies the pressurized oil to the solenoid valve mechanism 74 viathe remaining part of the oil passage 77 c.

The solenoid valve mechanism 74 is formed by combining a spool valvemechanism 74 a and a solenoid 74 b, and is connected to the advancechambers 75 and the retard chambers 76 via an advance oil passage 77 aand a retard oil passage 77 b such that oil pressure supplied from theoil pressure pump 73 is delivered to the advance chambers 75 and theretard chambers 76 as advance oil pressure Pad and retard oil pressurePrt, respectively. The solenoid 74 b of the solenoid valve mechanism 74is electrically connected to the ECU 2. When a phase control inputU_Cain, described hereinafter, is input from the ECU 2, the solenoid 74b moves a spool valve element of the spool valve mechanism 74 a within apredetermined range of motion according to the phase control inputU_Cain to thereby change both the advance oil pressure Pad and theretard oil pressure Prt.

In the variable cam phase mechanism 70 configured as above, duringoperation of the oil pressure pump 73, the solenoid valve mechanism 74is operated according to the phase control input U_Cain, to supply theadvance oil pressure Pad to the advance chambers 75 and the retard oilpressure Prt to the retard chambers 76, whereby the relative phase ofthe vane 72 with respect to the housing 71 is changed toward an advancedside or a retarded side. As a result, the cam phase Cain described aboveis continuously changed between a most retarded value Cainrt (valuecorresponding to a cam angle of e.g. 0°) and a most advanced valueCainad (value corresponding to a cam angle of e.g. 55°), whereby thevalve timing of the intake valves 4 is continuously changed between mostretarded timing indicated by a solid line in FIG. 9 and most advancedtiming indicated by a two-dot chain line in FIG. 9.

It should be noted that the variable cam phase mechanism 70 includes alock mechanism, not shown, which locks the operation of the variable camphase mechanism 70, when oil pressure supplied from the oil pressurepump 73 is low, when the phase control input U_Cain is set to afailure-time value U_Cain_fs, described hereinafter, or when the phasecontrol input U_Cain is not input to the solenoid valve mechanism 74e.g. due to a disconnection. That is, the variable cam phase mechanism70 is inhibited from changing the cam phase Cain, whereby the cam phaseCain is held at the predetermined locked value. The predetermined lockedvalue is set to such a value as will make it possible to ensure thepredetermined failure-time value Gcyl_fs of the intake air amount whenthe valve lift Liftin is held at the predetermined locked value, asdescribed above.

As described above, in the variable intake valve-actuating mechanism 40of the present embodiment, the variable valve lift mechanism 50continuously changes the valve lift Liftin between the maximum valueLiftinmax thereof and 0, and the variable cam phase mechanism 70continuously changes the cam phase Cain, i.e. the valve timing of theintake valves 4 between the most retarded timing and the most advancedtiming, described hereinbefore. Further, as described hereinafter, theECU 2 controls the valve lift Liftin and the cam phase Cain via thevariable valve lift mechanism 50 and the variable cam phase mechanism70, whereby the intake air amount is controlled.

On the other hand, a cam angle sensor 26 (see FIG. 2) is disposed at anend of the intake camshaft 5 opposite from the variable cam phasemechanism 70. The cam angle sensor 26 is implemented e.g. by a magnetrotor and an MRE pickup, for delivering a CAM signal, which is a pulsesignal, to the ECU 2 along with rotation of the intake camshaft 5. Eachpulse of the CAM signal is generated whenever the intake camshaft 5rotates through a predetermined cam angle (e.g. 1°). The ECU 2calculates the cam phase Cain based on the CAM signal and the CRKsignal, described above. It should be noted that in the presentembodiment, the cam angle sensor 26 corresponds to the first referenceparameter-detecting means, and the cam phase Cain corresponds to thefirst reference parameter.

Next, as shown in FIG. 2, connected to the ECU 2 are an acceleratorpedal opening sensor 27, and an ignition switch (hereinafter referred toas “the IG·SW”) 28. The accelerator pedal opening sensor 27 detects astepped-on amount AP of an accelerator pedal, not shown, of the vehicle(hereinafter referred to as “the accelerator pedal opening AP”) anddelivers a signal indicative of the sensed accelerator pedal opening APto the ECU 2. Further, the IG·SW 28 is turned on or off by operation ofan ignition key, not shown, and delivers a signal indicative of theON/OFF state thereof to the ECU 2.

The ECU 2 is implemented by a microcomputer comprised of a CPU, a RAM, aROM and an I/O interface (none of which are specifically shown). The ECU2 determines operating conditions of the engine 3, based on the signalsfrom the aforementioned sensors 20 to 27 and the ON/OFF signal from theIG·SW 28, and executes the control processes. More specifically, the ECU2 executes air-fuel ratio control and ignition timing control, accordingto the operating conditions of the engine 3, as described hereinafter.In addition, the ECU 2 calculates a corrected valve lift Liftin_mod, andcontrols the valve lift Liftin and the cam phase Cain via the variablevalve lift mechanism 50 and the variable cam phase mechanism 70, tothereby control the intake air amount.

It should be noted that in the present embodiment, the ECU 2 correspondsto the controlled variable-detecting means, the referenceparameter-detecting means, target value-setting means, controlinput-calculating means, error parameter-calculating means, influencedegree parameter-calculating means, corrected errorparameter-calculating means, model-modifying means, first inputvalue-calculating means, the first reference parameter-detecting means,the second reference parameter-detecting means, modificationvalue-calculating means, first influence degree parameter-calculatingmeans, corrected modification value-calculating means, and secondinfluence degree parameter-calculating means.

Next, a description will be given of the control apparatus 1 accordingto the present embodiment. The control apparatus 1, as shown in FIG. 10,includes an air-fuel ratio controller 100 which performs the air-fuelratio control. As will be described hereinafter, the air-fuel ratiocontroller 100 is provided for calculating the fuel injection amountTOUT for each fuel injection valve 10, and implemented by the ECU 2. Itshould be noted that in the present embodiment, the air-fuel ratiocontroller 100 corresponds to the control input-calculating means, andthe fuel injection amount TOUT to a control input and the amount of fuelto be supplied to the engine.

As shown in FIG. 10, the air-fuel ratio controller 100 includes firstand second estimated intake air amount-calculating sections 101 and 102,a transition coefficient-calculating section 103, amplification elements104 and 105, an addition element 106, an amplification element 107, atarget air-fuel ratio-calculating section 108, an air-fuel ratiocorrection coefficient-calculating section 109, a total correctioncoefficient-calculating section 110, a multiplication element 111, afuel attachment-dependent correction section 112, an air-fuel ratioerror estimated value-calculating section 113, an addition element 114,and a lift correction value-calculating section 120.

First, as described hereinafter, the first estimated intake airamount-calculating section 101 calculates a first estimated intake airamount Gcyl_vt. More specifically, first, the first estimated intake airamount-calculating section 101 calculates a basic estimated intake airamount Gcyl_vt_base by searching a map shown in FIG. 11, according tothe engine speed NE and the corrected valve lift Liftin_mod. Thecorrected valve lift Liftin_mod is a value obtained by correcting thevalve lift Liftin using a lift correction value Dlift, describedhereinafter. The reason for using the corrected valve lift Liftin_modfor calculating the first estimated intake air amount Gcyl_vt will bedescribed hereinafter. It should be noted that the first estimatedintake air amount-calculating section 101 uses a downsampled value asthe corrected valve lift Liftin_mod. Further, in FIG. 11, NE 1 to NE3represent predetermined values of the engine speed NE, which satisfy therelationship of NE1<NE2<NE3. This also applies to the followingdescription.

In this map, when NE=NE1 or NE2 holds, in a region where the correctedvalve lift Liftin_mod is small, the basic estimated intake air amountGcyl_vt_base is set to a larger value as the corrected valve liftLiftin_mod is larger, whereas in a region where the corrected valve liftLiftin_mod is close to the maximum value Liftinmax, the basic estimatedintake air amount Gcyl_vt_base is set to a smaller value as thecorrected valve lift Liftin_mod is larger. This is because in alow-to-medium engine speed region, as the corrected valve liftLiftin_mod is larger in the region where the corrected valve liftLiftin_mod is close to the maximum value Liftinmax, the valve-openingtime period of the intake valve 4 becomes longer, whereby chargingefficiency is reduced by blow-back of intake air. Further, when NE=NE3holds, the basic estimated intake air amount Gcyl_vt_base is set to alarger value as the corrected valve lift Liftin_mod is larger. This isbecause in a high engine speed region, the above-described blow-back ofintake air is made difficult to occur even in a region where thecorrected valve lift Liftin_mod is large, due to the inertia force ofintake air, so that the charging efficiency becomes higher as thecorrected valve lift Liftin_mod is larger.

Further, a correction coefficient K_gcyl_vt is calculated by searching amap shown in FIG. 12, according to the engine speed NE and the cam phaseCain. In this map, when NE=NE1 or NE2 holds, in a region where the camphase Cain is close to the most retarded value Cainrt, the correctioncoefficient K_gcyl_vt is set to a smaller value as the cam phase Cain iscloser to the most retarded value Cainrt, and in the other regions, thecorrection coefficient K_gcyl_vt is set to a smaller value as the camphase Cain takes a value closer to the most advanced value Cainad. Thisis because in the low-to-medium engine speed region, as the cam phaseCain is closer to the most retarded value Cainrt in the region where thecam phase Cain is close to the most retarded value Cainrt, thevalve-closing timing of the intake valve 4 is retarded, whereby thecharging efficiency is degraded by the blow-back of intake air, and inthe other regions, as the cam phase Cain takes a value closer to themost advanced value Cainad, the valve overlap increases to increase theinternal EGR amount, whereby the charging efficiency is degraded.Further, when NE=NE3 holds, in the region where the cam phase Cain isclose to the most retarded value Cainrt, the correction coefficientK_gcyl_vt is set to a fixed value (a value of 1), and in the otherregions, the correction coefficient K_gcyl_vt is set to a smaller valueas the cam phase Cain takes a value closer to the most advanced valueCainad. This is because in the high engine speed region, the blow-backof intake air is made difficult to occur even in a region where the camphase Cain is close to the most advanced value Cainad, due to theabove-mentioned inertia force of intake air.

Then, the first estimated intake air amount Gcyl_vt is calculated usingthe basic estimated intake air amount Gcyl_vt_base and the correctioncoefficient K_gcyl_vt, calculated as above, by the following equation(1):Gcyl _(—) vt(n)=K _(—) gcyl _(—) vt(n)·Gcyl _(—) vt_base(n)  (1)

In the above equation (1), discrete data with a symbol (n) indicatesthat it is data sampled or calculated at a control period ΔTnsynchronous with generation of each TDC signal pulse. The symbol nindicates a position in the sequence of sampling or calculating cyclesof respective discrete data. For example, the symbol n indicates thatdiscrete data therewith is a value sampled in the current controltiming, and a symbol n−1 indicates that discrete data therewith is avalue sampled in the immediately preceding control timing. It should benoted that in the following description, the symbol (n) and the likeprovided for the discrete data are omitted as deemed appropriate.

Now, the method of calculating the first estimated intake air amountGcyl_vt in the first estimated intake air amount-calculating section 101is not limited to the above-described method, but any suitable methodmay be employed insofar as it calculates the first estimated intake airamount Gcyl_vt according to the engine speed NE, the corrected valvelift Liftin_mod, and the cam phase Cain. For example, the firstestimated intake air amount Gcyl_vt may be calculated using a4-dimensional map in which the relationship between the first estimatedintake air amount Gcyl_vt, the engine speed NE, the corrected valve liftLiftin_mod, and the cam phase Cain is set in advance. Further, the firstestimated intake air amount Gcyl_vt may be calculated using a neuralnetwork to which are input the engine speed NE, the corrected valve liftLiftin_mod, and the cam phase Cain, and from which is output the firstestimated intake air amount Gcyl_vt.

It should be noted that in the present embodiment, the first estimatedintake air amount-calculating section 101 corresponds to the first inputvalue-calculating means, and the first estimated intake air amountGcyl_vt corresponds to the first input value.

Further, the transition coefficient-calculating section 103 calculates atransition coefficient Kg as follows: First, an estimated flow rateGin_vt (in units of g/sec) is calculated by the following equation (2),using the first estimated intake air amount Gcyl_vt calculated by thefirst estimated intake air amount-calculating sections 101, and theengine speed NE.

$\begin{matrix}{{{Gin\_ vt}(n)} = \frac{{2 \cdot {Gcyl\_ vt}}{(n) \cdot {{NE}(n)}}}{60}} & (2)\end{matrix}$

Subsequently, the transition coefficient Kg is calculated by searching atable shown in FIG. 13 according to the estimated flow rate Gin_vt. InFIG. 13, Gin1 and Gin2 represent predetermined values which satisfy therelationship of Gin1<Gin2. Since the flow rate of air flowing throughthe intake passage 12 a is small when the estimated flow rate Gin_vt iswithin the range of the Gin_vt≦Gin1, the predetermined value Gin1 is setto such a value as will cause the reliability of the first estimatedintake air amount Gcyl_vt to exceed that of a second estimated intakeair amount Gcyl_afm, referred to hereinafter, due to the resolution ofthe air flow sensor 22. Further, since the flow rate of air flowingthrough the intake passage 12 a is large when the estimated flow rateGin_vt is within the range of Gin2≦Gin_vt, the predetermined value Gin2is set to such a value as will cause the reliability of the secondestimated intake air amount Gcyl_afm to exceed that of the firstestimated intake air amount Gcyl_vt. Furthermore, in this table, thetransition coefficient Kg is set to 0 when the first estimated intakeair amount Gcyl_vt is in the range of Gin_vt≦Gin1, and to 1 when thesame is within the range of Gin2≦Gin_vt. When the estimated flow rateGin_vt is within the range of Gin1<Gin_vt<Gin2, the transitioncoefficient Kg is set to a value which is between 0 and 1, and at thesame time larger as the estimated flow rate Gin_vt is larger.

On the other hand, the second estimated intake air amount-calculatingsection 102 calculates the second estimated intake air amount Gcyl_afm(unit: g) based on the air flow rate Gin and the engine speed NE, by thefollowing equation (3):

$\begin{matrix}{{{Gcyl\_ afm}(n)} = \frac{{{Gin}(n)} \cdot 60}{2 \cdot {{NE}(n)}}} & (3)\end{matrix}$

The amplification elements 104 and 105 amplify the first and secondestimated intake air amounts Gcyl_vt and Gcyl_afm, calculated as above,to a (1-Kg)-fold and a Kg-fold, respectively. The addition element 106calculates a calculated intake air amount Gcyl based on the values thusamplified, by a weighted average arithmetic operation expressed by thefollowing equation (4):Gcyl(n)=Kg·Gcyl _(—) afm(n)+(1−Kg)·Gcyl _(—) vt(n)  (4)

As is clear from the equation (4), when Kg=0, i.e. within theaforementioned range of Gin_vt≦Gin1, Gcyl=Gcyl_vt holds, and when Kg=1,i.e. within the aforementioned range of Gin2≦Gin_vt, Gcyl=Gcyl_afmholds. When 0<Kg<1, i.e. when the estimated flow rate Gin_vt is withinthe range of Gin1<Gin_vt<Gin2, the degrees of contributions of (thedegrees of weighting) the first and second estimated intake air amountsGcyl_vt and Gcyl_afm in the calculated intake air amount Gcyl aredetermined by the value of the transition coefficient Kg.

Furthermore, the amplification element 107 calculates a basic fuelinjection amount Tcyl_bs based on the calculated intake air amount Gcyl,by the following equation (5). It should be noted that in the followingequation (5), Kgt represents a conversion coefficient set in advance foreach fuel injection valve 10.Tcyl _(—) bs(n)=Kgt·Gcyl(n)  (5)

Further, the target air-fuel ratio-calculating section 108 calculates atarget air-fuel ratio KCMD by searching a map shown in FIG. 14 accordingto the calculated intake air amount Gcyl and the accelerator pedalopening AP. In this map, the value of the target air-fuel ratio KCMD isset as an equivalent ratio, and basically, it is set to a valuecorresponding to a stoichiometric air-fuel ratio (14.5) so as tomaintain excellent emission-reducing performance of the catalyticconverter. It should be noted that in the present embodiment, the targetair-fuel ratio-calculating section 108 corresponds to the targetvalue-setting means, and the target air-fuel ratio KCMD corresponds to atarget value.

Furthermore, the air-fuel ratio correction coefficient-calculatingsection 109 calculates an air-fuel ratio correction coefficient KAF witha sliding mode control algorithm expressed by the following equations(6) to (10). It should be noted that in the above equations (6) to (10),discrete data with a symbol (m) indicates that it is data sampled orcalculated every combustion cycle, i.e. whenever a total of foursuccessive pulses of the TDC signal are generated. The symbol mindicates a position in the sequence of sampling cycles of respectivediscrete data.KAF(m)=Urch′(m)+Uadp′(m)  (6)Urch′(m)=−Krch′·σ′(m)  (7)Uadp′(m)=Uadp′(m−1)−Kadp′·σ′(m)  (8)σ′(m)=e(m)+S′·e(m−1)  (9)e(m)=KACT(m)−KCMD(m)  (10)

As shown in the equation (6), the air-fuel ratio correction coefficientKAF is calculated as the sum of a reaching law input Urch′ and anadaptive law input Uadp′ and the reaching law input Urch′ is calculatedusing the equation (7). In the equation (7), Krch′ represents apredetermined reaching law gain, and σ′ represents a switching functiondefined by the equation (9). In the equation (9), S′ represents aswitching function-setting parameter set to a value which satisfies therelationship of −1<S′<0 and the symbol e represents a follow-up errordefined by the equation (10). In this case, the convergence rate of thefollow-up error “e” to 0 is designated by a value set to the switchingfunction-setting parameter S′.

Furthermore, the adaptive law input Uadp′ is calculated by the equation(8). In the equation (8), Kadp′ represents a predetermined adaptive lawgain. It should be noted that the initial value of the adaptive lawinput Uadp′ is set to 1.

As described above, the air-fuel ratio correctioncoefficient-calculating section 109 calculates the air-fuel ratiocorrection coefficient KAF as a value for causing the actual air-fuelratio KACT to converge to the target air-fuel ratio KCMD, with thesliding mode control algorithm expressed by the following equations (6)to (10). It should be noted that in the present embodiment, the air-fuelratio correction coefficient KAF corresponds to a second input value.

On the other hand, the total correction coefficient-calculating section110 calculates various correction coefficients by searching respectiveassociated maps, not shown, according to parameters, such as the enginecoolant temperature TW and the intake air temperature TA, indicative ofthe operating conditions of the engine, and calculates a totalcorrection coefficient KTOTAL by multiplying the thus calculatedcorrection coefficients by each other.

Further, the multiplication element 111 calculates a demanded fuelinjection amount Tcyl by the following equation (11):Tcyl(n)=Tcyl _(—) bs(n)·KAF(n)·KTOTAL(n)  (11)

Furthermore, the fuel attachment-dependent correction section 112calculates the fuel injection amount TOUT by performing a predeterminedfuel attachment-dependent correction process on the demanded fuelinjection amount Tcyl calculated as above. Then, the fuel injectionvalve 10 is controlled such that the fuel injection timing and thevalve-opening time period thereof are determined based on the fuelinjection amount TOUT.

Next, a description will be given of the air-fuel ratio error estimatedvalue-calculating section 113. As described hereinafter, the air-fuelratio error estimated value-calculating section 113 calculates anair-fuel ratio error estimated value Eaf. First, the air-fuel ratioerror estimated value-calculating section 113 calculates an actualair-fuel ratio estimated value KACT_hat based on the air-fuel ratiocorrection coefficient KAF and the actual air-fuel ratio KACT, by thefollowing equation (12), and then calculates the air-fuel ratio errorestimated value Eaf by the following equation (13).

$\begin{matrix}{{{KACT\_ hat}(k)} = \frac{{KACT}(k)}{{KAF}( {k - d} )}} & (12) \\{{{Eaf}\mspace{11mu}(k)} = {{{KACT\_ hat}(k)} - {{KCMD}\mspace{11mu}( {k - d} )}}} & (13)\end{matrix}$

In the above equations (12) and (13), discrete data with a symbol (k)indicates that it is data sampled or calculated at a predeterminedcontrol period ΔTk (5 msec, in the present embodiment). The symbol kindicates a position in the sequence of sampling or calculating cyclesof respective discrete data. It should be noted that in the followingdescription, the symbol (k) provided for the discrete data is omitted asdeemed appropriate. Further, in the above equations (12) and (13), asymbol “d” represents a dead time it takes for combustion gases to reachthe LAF sensor 24 from the combustion chamber.

As shown in the equation (12), the actual air-fuel ratio estimated valueKACT_hat is calculated by dividing an actual air-fuel ratio KACT(k)obtained in the current control timing by an air-fuel ratio correctioncoefficient KAF(k−d) calculated in control timing the dead time dearlier, and hence as a value which is not adversely affected by theair-fuel ratio correction coefficient KAF(k−d). More specifically, theactual air-fuel ratio estimated value KACT_hat is calculated as a valueof the actual air-fuel ratio in the current control timing, estimatedassuming that air-fuel ratio feedback control was not executed in thecontrol timing the dead time d earlier.

Therefore, the air-fuel ratio error estimated value Eaf is calculated asthe difference between the actual air-fuel ratio estimated valueKACT_hat(k) calculated as above and a target air-fuel ratio KCMD(k−d)calculated in control timing the dead time d earlier, and hence theair-fuel ratio error estimated value Eaf corresponds to an error ofair-fuel ratio control in the current control timing, estimated assumingthat the air-fuel ratio feedback control was not executed in the controltiming the dead time d earlier. It should be noted that in the presentembodiment, the air-fuel ratio error estimated value-calculating section113 corresponds to the error parameter-calculating means, and theair-fuel ratio error estimated value Eaf corresponds to an errorparameter.

Next, a description will be given of the aforementioned lift correctionvalue-calculating section 120. The lift correction value-calculatingsection 120 calculates a lift correction value Dlift by a method,described hereinafter. As described hereinbefore, in the controlapparatus 1, the basic estimated intake air amount Gcyl_vt_base iscalculated using the corrected valve lift Liftin_mod obtained bycorrecting the valve lift Liftin by the lift correction value Dlift, andthe FIG. 11 map. Hereinafter, the reason for using the corrected valvelift Liftin_mod will be described.

When the intake air amount is controlled via the variable valve liftmechanism 50 as in the control apparatus 1 of the present embodiment,the relationship between the valve lift Liftin and the basic estimatedintake air amount Gcyl_vt_base deviates from the actual relationshiptherebetween, whereby when the basic estimated intake air amountGcyl_vt_base is calculated using the valve lift Liftin (e.g. when thevalve lift Liftin is represented by the horizontal axis in FIG. 11,referred to hereinabove), there is a possibility that the calculatedvalue of the basic estimated intake air amount Gcyl_vt_base is differentfrom the actual value thereof.

More specifically, when the mounted state of the pivot angle sensor 25is changed e.g. by impact, or the characteristic of the pivot anglesensor 25 changes with a change in the temperature thereof, thecalculated value of the valve lift Liftin sometimes deviates from theactual value thereof, and in such a case, there occurs an error in thecalculation of the aforementioned basic estimated intake air amountGcyl_vt_base. Further, also when the dynamic characteristics of thevariable valve lift mechanism 50 (i.e. the relationship of the valvelift Liftin to the lift control input U_Liftin) are changed by wear ofcomponents of the variable valve lift mechanism 50, attachment of stain,and play produced by aging, there occurs an error in the calculation ofthe basic estimated intake air amount Gcyl_vt_base. In the followingdescription, a state where the relationship between the valve liftLiftin and the basic estimated intake air amount Gcyl_vt_base hasdeviated from the actual relationship therebetween is referred to as“the lift error”.

It is considered that when the engine speed NE is a low engine speedregion, the state where the above lift error occurs includes those shownin FIGS. 15 and 16. FIG. 15 shows a state in which the above-describedlift error has occurred due to the offset (zero-point deviation) of thecalculated value of the valve lift Liftin with respect to the actualvalue thereof. Further, FIG. 16 shows a state in which the lift errorhas occurred due to the aforementioned change in the dynamiccharacteristics of the variable valve lift mechanism 50, although thereis no error between the calculated value of the valve lift Liftin andthe actual value thereof. In FIGS. 15 and 16, curves indicated by solidlines indicate states in which the lift error occurs in the relationshipbetween the valve lift Liftin and the basic estimated intake air amountGcyl_vt_base, and curves indicated by broken lines show states in whichthe lift error occurs.

As is clear from FIGS. 15 and 16, the lift error becomes larger when thevalve lift Liftin is equal to a predetermined value Liftin_a in a smalllift region than when the valve lift Liftin is equal to a predeterminedvalue Liftin_b in a large lift region. More specifically, it isunderstood that the lift error becomes larger in the small lift regionthan in the large lift region both when the lift error occurs due to theabove-described offset of the valve lift Liftin and when it occurs dueto the dynamic characteristics of the variable valve lift mechanism 50.

Further, as is clear from FIG. 17, when a change amount ΔGcyl of thebasic estimated intake air amount Gcyl_vt_base with respect to a changeamount ΔLiftin of the valve lift Liftin is considered, a value ΔGcyl_athereof in the small lift region is larger than a value ΔGcyl_b thereofin the large lift region, so that a ratioΔGcyl/ΔLiftin between the twochange amounts satisfies the relationship of (ΔGcyl_a/ΔLiftin)

(ΔGcyl_b/ΔLiftin).

Now, assuming that the air-fuel ratio error estimated value Eaf isgenerated due to the lift error, the degree of influence of the lifterror on the air-fuel ratio error estimated value Eaf, that is, thesensitivity of the air-fuel ratio error estimated value Eaf to the lifterror can be considered to increase or decrease in the same manner asthe magnitude of the above-described ratio ΔGcyl/ΔLiftin. In otherwords, when the air-fuel ratio error estimated value Eaf is generated,it can be considered that the probability of the air-fuel ratio errorestimated value Eaf being caused by the lift error is higher as theratio ΔGcyl/ΔLiftin is larger. Furthermore, the value of the ratioΔGcyl/ΔLiftin changes not only according to the valve lift Liftin andthe engine speed NE (see FIG. 11, referred to hereinabove) but alsoaccording to the cam phase Cain, and hence the sensitivity of theair-fuel ratio error estimated value Eaf to the lift error also changesaccording to the three values of Liftin, Ne, and Cain.

For the above reason, the lift correction value-calculating section 120calculates the lift correction value Dlift for correcting the valve liftLiftin by a method, described hereinafter, as a value which reflects theabove-described sensitivity of the air-fuel ratio error estimated valueEaf to the lift error.

As shown in FIG. 18, the lift correction value-calculating section 120is comprised of an error weight-calculating section 121, a modifiederror-calculating section 122, a basic lift correction value-calculatingsection 123, a correction sensitivity-calculating section 124, amultiplication element 125, and an addition element 126. It should benoted that in the present embodiment, the lift correctionvalue-calculating section 120 corresponds to the model-modifying meansand the corrected modification value-calculating means, and the liftcorrection value Dlift corresponds to a corrected modification value.

First, the error weight-calculating section 121 calculates an errorweight W, as described hereinafter. It should be noted that in thepresent embodiment, the error weight-calculating section 121 correspondsto the influence degree parameter-calculating means and the secondinfluence degree parameter-calculating means, and the error weight Wcorresponds to an influence degree parameter and a second influencedegree parameter.

First, the error weight-calculating section 121 calculates a secondcorrected valve lift Liftin_mod_p by the following equation (14):Liftin_mod_(—) p(k)=Liftin(k)+Dlift(k−1)  (14)

As shown in the equation (14), the second corrected valve liftLiftin_mod_p is calculated as the sum of the current value Liftin(k) ofthe valve lift and the immediately preceding value Dlift(k−1) of thelift correction value. This is because the current value Dlift(k) of thelift correction value has not been calculated yet when the secondcorrected valve lift Liftin_mod_p is calculated.

Then, the error weight-calculating section 121 calculates a basic errorweight W_base by searching a map shown in FIG. 19 according to thesecond corrected valve lift Liftin_mod_p and the engine speed NE. Thebasic error weight W_base takes a value obtained by normalizing theaforementioned ratio ΔGcyl/ΔLiftin with reference to the absolute value|ΔGcyl_x/ΔLiftin_x of a ratio ΔGcyl_x/ΔLiftin_x obtained at apredetermined minute lift and a predetermined low engine speed, that is,a value which satisfies the equation,W_base=(ΔGcyl/ΔLiftin)÷(|ΔGcyl_x/ΔLiftin_x|). As shown by broken linesin FIG. 21, on condition that ΔGcyl/ΔLiftin<0 holds, the basic errorweight W_base is set to 0 for the reason described hereinafter.

In this map, the basic error weight W_base is set to a larger value asthe second corrected valve lift Liftin_mod_p is smaller. This is becausethe aforementioned sensitivity of the air-fuel ratio error estimatedvalue Eaf to the lift error, i.e. the ratio ΔGcyl/ΔLiftin takes a largervalue as the second corrected valve lift Liftin_mod_p is smaller.Further, in the small lift region, the basic error weight W_base is setto a smaller value as the engine speed NE is higher, whereas in theother lift regions, the basic error weight W_base is set to a largervalue as the engine speed NE is higher. The reason for this is the sameas given in the description of the FIG. 11 map (changes in the chargingefficiency and the blow-back of intake air). It should be noted that inthe present embodiment, the map shown in FIG. 19 corresponds to aninfluence degree model and a second influence degree model.

Further, the error weight-calculating section 121 calculates an errorweight correction coefficient K_w by searching a map shown in FIG. 20according to the cam phase Cain and the engine speed NE. The errorweight correction coefficient K_w takes a value obtained by normalizingthe aforementioned ratio ΔGcyl/ΔLiftin with reference to the absolutevalue ΔGcyl_rt/ΔLiftin_rt of a ratio ΔGcyl_rt/ΔLiftin_rt obtained whenthe cam phase Cain is equal to the most retarded value, at each of thepredetermined values NE1 to NE3 of the engine speed NE, that is, a valuewhich satisfies the equation,W_base=(ΔGcyl/ΔLiftin)÷(|ΔGcyl_rt/ΔLiftin_rt|).

In this map, the error weight correction coefficient K_w is set to havethe same tendency as that of the FIG. 12 correction coefficientK_gcyl_vt described above, with respect to the engine speed NE and thecam phase Cain. The reason for this is the same as given in thedescription of the FIG. 12 map (changes in the charging efficiency andthe blow-back of intake air).

Then, finally, the error weight W is calculated by the followingequation (15).W(k)=W_base(k)·K _(—) w(k)  (15)

Thus, the error weight W is calculated by multiplying the basic errorweight W_base by the error weight correction coefficient K_w, and hencethe error weight W is calculated as a value which represents thesensitivity of the air-fuel ratio error estimated value Eaf to the lifterror. More specifically, the error weight W is calculated as a largervalue as the sensitivity of the air-fuel ratio error estimated value Eafto the lift error, i.e. the ratio ΔGcyl/ΔLiftin is larger, in otherwords, as the probability of the air-fuel ratio error estimated valueEaf being caused by the lift error is higher. Further, the two valuesW_base and K_w are calculated by searching the two maps shown in FIGS.19 and 20 according to the three parameters Liftin_mod_p, NE, and Cain,and the second corrected valve lift Liftin_mod_p is a value obtained byadding the immediately preceding value Dlift(k−1) of the lift correctionvalue to the valve lift, so that it can be considered that the above twomaps form a response surface model which represents the correlationbetween the three values Liftin, NE, and Cain, and the error weight W.

Thus, the error weight W is calculated according to the three valuesLiftin, NE, and Cain since the sensitivity of the air-fuel ratio errorestimated value Eaf to the lift error is changed not only by the valueof the valve lift Liftin but also by the values of the engine speed NEand the cam phase Cain. As a result, the error weight W is calculated asa value which represents the degree of influence of the three valuesLiftin, NE, and Cain on the air-fuel ratio error estimated value Eaf.

It should be noted that the FIG. 19 map for use in calculating the basicerror weight W_base may be replaced by a map in which the basic errorweight W_base is set according to the valve lift Liftin and the enginespeed NE, that is, a map in which the second corrected valve liftLiftin_mod_p represented by the horizontal axis in FIG. 19 is replacedby the valve lift Liftin.

The modified error-calculating section 122 calculates a modified errorWeaf by the following equation (16). It should be noted that themodified error-calculating section 122 corresponds to the correctederror parameter-calculating means, and the modified error Weafcorresponds to a corrected error parameter.Weaf(k)=W(k)·(1−Kg(k−d))·Eaf(k)  (16)

In the above equation (16), a transition coefficient Kg(k−d) the deadtime d earlier is used for the following reason: As is clear fromreference to the aforementioned equation (4), when the transitioncoefficient Kg changes, the degrees of contributions of the firstestimated intake air amount Gcyl_vt and the second estimated intake airamount Gcyl_afm in the calculated intake air amount Gcyl also change tochange the sensitivity of the air-fuel ratio error estimated value Eafto the lift error. In this case, the air-fuel ratio error estimatedvalue Eaf calculated in the current control timing is caused by acalculated intake air amount Gcyl(k−d) calculated in control timing thedead time d earlier and the fuel injection amount TOUT calculated basedon the calculated intake air amount Gcyl(k−d), so that it is assumedthat the change in the sensitivity of the air-fuel ratio error estimatedvalue Eaf to the lift error in the current control timing is caused by achange in the transition coefficient Kg(k−d) the dead time d earlier.Therefore, to compensate for the change in the sensitivity of theair-fuel ratio error estimated value Eaf to the lift error, thetransition coefficient Kg(k−d) the dead time d earlier is employed forcalculation of the transition coefficient weight Wkg.

Further, the basic lift correction value-calculating section 123calculates a basic lift correction value Dlift_bs with a controlalgorism to which is applied a sliding mode control algorithm expressedby the following equations (17) to (24). That is, the basic liftcorrection value Dlift_bs is calculated as a value for causing themodified error Weaf to converge to 0.σ(k)=Weaf(k)+S·Weaf(k−1)  (17)Urch(k)=−Krch·σ(k)  (18)Unl(k)=−Knl·sgn(σ(k))  (19)Uadp(k)=−Kadp·δ(k)  (20)δ(k)=λ·δ(k−1)+σ(k)  (21)When Dlift_(—) bs _(—) L<Dlift_(—) bs(k−1)<Dlift_(—) bs _(—) H λ=1  (22)When Dlift_(—) bs(k−1)≦Dlift_(—) bs _(—) LorDlift_(—) bs _(—)H≦Dlift_(—) bs(k−1) λ=λlmt  (23)Dlift_(—) bs(k)=Urch(k)+Unl(k)+Uadp(k)  (24)

In the above equation (17), σ represents a switching function, and Srepresents a switching function-setting parameter set to a value whichsatisfies the relationship of −1<S<0. In this case, the convergence rateof the modified error Weaf to 0 is designated by a value set to theswitching function-setting parameter S. Further, in the equation (18),Urch represents a reaching law input, and Krch a predetermined reachinglaw gain. Furthermore, in the equation (19), Unl represents a non-linearinput, and Knl a predetermined non-linear input gain. Further, in theequation (19), sgn(a (k)) represents a sign function, and the valuethereof is set such that sgn(σ(k))=1 holds when σ(k)≧0, and when σ(k)<0,sgn(σ(k))=−1 holds (it should be noted that the value thereof may be setsuch that sgn(σ(k))=0 holds when σ(k)=0).

In the equation (20), Uadp represents an adaptive law input, and Kadprepresents a predetermined adaptive law gain. Further, in the equation(20), δ represents the integral value of a switching function calculatedby the equation (21). In the equation (21), λ represents a forgettingcoefficient, and as shown in the equations (22) and (23), the valuethereof is set to 1 or a predetermined value λlmt, according to theresults of comparison between the immediately preceding valueDlift_bs(k−1) of the basic lift correction value and predetermined upperand lower limit values Dlift_bs_H and Dlift_bs_L. The upper limit valueDlift_bs_H is set to a predetermined positive value, and the lower limitvalue Dlift_bs_L is set to a predetermined negative value, while thepredetermined value λlmt is set to a value which satisfies therelationship of 0<λlmt<1.

Further, as shown in the equation (24), the basic lift correction valueDlift_bs is calculated as the sum of the reaching law input Urch, thenon-linear input Unl, and the adaptive law input Uadp.

The forgetting coefficient λ is used in the algorithm for calculatingthe basic lift correction value Dlift_bs for the following reason: Theair-fuel ratio correction coefficient KAF is calculated with the slidingmode control algorithm expressed by the equations (6) to (10), and thebasic lift correction value Dlift_bs is calculated with the controlalgorithm to which is applied the sliding mode control algorithmexpressed by the equations (17) to (24), such that the modified errorWeaf calculated based on the air-fuel ratio correction coefficient KAFconverges to 0. Therefore, unless the forgetting coefficient λ is used,the adaptive law inputs Uadp′ and Uadp as integral terms in the abovetwo control algorithms interfere with each other to exhibit anoscillating behavior, or the absolute values of the respective adaptivelaw inputs increase (i.e. the same state as in the parameter driftduring the adaptive control), whereby the basic lift correction valueDlift_bs, i.e. the calculated value of the first estimated intake airamount Gcyl_vt temporarily becomes improper, which degradescontrollability in a transient state.

In contrast, in the aforementioned equation (21), when the absolutevalue of the immediately preceding value Dlift_bs(k−1) of the basic liftcorrection value is large, to avoid an increase in the integral value δof the switching function of the adaptive law input Uadp, theimmediately preceding value δ (k−1) of the integral value of theswitching function is multiplied by the forgetting coefficient λ whichis set to a value within a range of 0<λ<1. In this case, when theaforementioned equation (21) is expanded by a recurrence formulathereof, the integral value δ (k−h) of the switching function calculatedin control timing h (h is a natural number not smaller than 2) timesearlier is multiplied by λ^(h) (≈0), so that even when the calculationprocess proceeds, it is possible to avoid an increase in the integralvalue δ of the switching function, that is, an increase in the adaptivelaw input Uadp. As a result, it is possible to prevent the firstestimated intake air amount Gcyl_vt from oscillating or temporarilytaking an improper value, thereby making it possible to improvecontrollability in a transient state.

Further, if the forgetting coefficient λ is always set to a value withinthe range of 0<λ<1, when the modified error Weaf takes a value close to0, the basic lift correction value Dlift_bs come to converge to a valueclose to 0 due to a forgetting effect provided by the forgettingcoefficient λ, so that when a control error occurs again in such astate, it takes time to eliminate the control error. Therefore, to avoidthe inconvenience and eliminate the control error quickly, it isnecessary to hold the basic lift correction value Dlift_bs at a valuecapable of compensating for the modified error Weaf even when the valueof the modified error Weaf is relatively small. Therefore, in thepresent embodiment, when the immediately preceding value Dlift_bs(k−1)of the basic lift correction value is within the above-described range,the forgetting coefficient λ is set to 1 so as to cancel the forgettingeffect provided by the forgetting coefficient λ. It should be noted thatwhen the forgetting effect by the forgetting coefficient λ is alwaysunnecessary, the forgetting coefficient λ may be set to 1 in theequation (21) irrespective of the magnitude of the immediately precedingvalue Dlift_bs(k−1).

Further, the basic lift correction value Dlift_bs is calculated by theaforementioned equations (17) to (24) such that the modified error Weafis caused to converge to 0, and hence e.g. when the above-describedbasic error weight W_base takes both a positive value and a negativevalue, if the basic error weight W_base changes between the positivevalue and the negative value, the sign of the modified error Weaf isinverted along with the change in the basic error weight W_base toinvert the signs of the respective control inputs Urch, Unl, and Uadp,whereby the basic lift correction value Dlift_bs is calculated as animproper value, which can make the control unstable. Therefore, toensure the stability of the control, in the above-described FIG. 19, thebasic error weight W_base is set to 0 under a condition where it takes anegative value.

It should be noted that when the signs of gains of the respectivecontrol inputs Urch, Unl, and Uadp are controlled to be inverted alongwith the change in the sign of the basic error weight W_base, even whenthe basic error weight W_base takes both a positive value and a negativevalue, it is possible to ensure the stability of control, similarly tothe present embodiment. Therefore, in such a case, the values of curves,shown by broken lines in FIG. 19, may be used on which the basic errorweight W_base takes negative values. It should be noted that in thepresent embodiment, the basic lift correction value-calculating section123 corresponds to the modification value-calculating means, and thebasic lift correction value Dlift_bs corresponds to a modificationvalue.

On the other hand, the above-described correctionsensitivity-calculating section 124 calculates a correction sensitivityRlift by the following method: First, the correctionsensitivity-calculating section 126 calculates a second corrected valvelift Liftin_mod_p by the aforementioned equation (14).

Then, the correction sensitivity-calculating section 126 calculates abasic sensitivity R_base by searching a map shown in FIG. 21 accordingto the second corrected valve lift Liftin_mod_p and the engine speed NE.Similarly to the above-described basic error weight W_base, the basicsensitivity R_base takes a value obtained by normalizing the ratioΔGcyl/ΔLiftin with reference to the absolute value |ΔGcyl_x/ΔLiftin_x|of the ratio ΔGcyl_x/ΔLiftin_x obtained at a predetermined minute liftand a predetermined low engine speed.

In this map, the basic sensitivity R_base is set to a larger value asthe second corrected valve lift Liftin_mod_p is smaller. The reason forthis is the same as given in the description of the FIG. 19 map.Further, in this map, differently from the basic error weight W_base,the basic sensitivity R_base is configured to assume both a positivevalue and a negative value. This is because as described hereinafter,the lift correction value Dlift is calculated by multiplying the basiclift correction value Dlift_bs by the correction sensitivity Rlift, andthe corrected valve lift Liftin_mod is calculated by adding the liftcorrection value Dlift to the valve lift Liftin, so that even when thecorrection sensitivity Rlift takes both a positive value and a negativevalue, it is possible to enhance the responsiveness of the air-fuelratio control without spoiling the stability of control.

Further, the correction sensitivity-calculating section 124 calculates asensitivity correction coefficient K_r by searching a map shown in FIG.22 according to the cam phase Cain and the engine speed NE. In FIG. 24,curves indicated by solid lines represent the values of the sensitivitycorrection coefficient K_r, and curves indicated by broken linesrepresent the values of the error weight correction coefficient K_w, forcomparison. As is clear from the comparison between the curves, in thismap, the sensitivity correction coefficient K_r is configured to haveapproximately the same tendency as that of the error weight correctioncoefficient K_w. The reason for this is the same as given in thedescription of the FIG. 20 map. In addition, the value of thesensitivity correction coefficient K_r on an advanced side thereof isset to a value closer to 1 than that of the error weight correctioncoefficient K_w. This is because when the cam phase Cain is controlledto be advanced, the fuel injection amount TOUT is calculated as asmaller value according to a decrease in the intake air amount, so thatwhen the fuel injection amount TOUT is erroneously calculated as a valuesmaller than an appropriate value, the stability of combustion can bedegraded by leaning of the air-fuel mixture. To avoid this problem, themap is configured as described above.

Then, finally, the correction sensitivity Rlift is calculated by thefollowing equation (25).Rlift(k)=R_base(k)·K _(—) r(k)  (25)

As described above, since the correction sensitivity Rlift is calculatedby the same method as employed for the calculation of the error weightW, the correction sensitivity Rlift is calculated not only as a valueindicative of the sensitivity of the air-fuel ratio error estimatedvalue Eaf to the lift error, that is, the degree of influence of thevalve lift Liftin on the air-fuel ratio error estimated value Eaf butalso as a value indicative of the degree of influence of the enginespeed NE and the cam phase Cain on the air-fuel ratio error estimatedvalue Eaf. Then, the lift correction value Dlift is calculated bymultiplying the basic lift correction value Dlift_bs by the correctionsensitivity Rlift calculated as above. The lift correction value Dliftis calculated by multiplying the basic lift correction value Dlift_bs bythe correction sensitivity Rlift, as described above, since there is afear that a change in the air-fuel ratio error estimated value Eaf isovercompensated for by the lift correction value Dlift if the liftcorrection value Dlift is calculated such that Dlift_bs=Dlift holds,without using the correction sensitivity Rlift under a condition wherethe sensitivity of the air-fuel ratio error estimated value Eaf to thelift error is low. To avoid this problem, the lift correction valueDlift is calculated as above.

It should be noted that as a map for use in calculating the basicsensitivity R_base, the correction sensitivity-calculating section 124may use, in place of the FIG. 21 map, a map in which the basicsensitivity R_base is set according to the valve lift Liftin and theengine speed NE, that is, a map in which the second corrected valve liftLiftin_mod_p represented by the horizontal axis in FIG. 21 is replacedby the valve lift Liftin. It should be noted that in the presentembodiment, the correction sensitivity-calculating section 124corresponds to the first influence degree parameter-calculating means,the correction sensitivity Rlift corresponds to the first influencedegree parameter, and the maps shown in FIGS. 21 and 22 correspond tothe first influence degree models.

Subsequently, the multiplication element 125 calculates the liftcorrection value Dlift by the following equation (26).Dlift(k)=Rlift(k)·Dlift_(—) bs(k)  (26)

The lift correction value-calculating section 120 calculates the liftcorrection value Dlift by the above-described method Then, theaforementioned addition element 114 calculates the corrected valve liftLiftin_mod by the following equation (27):Liftin_mod(k)=Liftin(k)+Dlift(k)  (27)

As described above, the lift correction value Dlift is calculated bymultiplying the basic lift correction value Dlift_bs by the correctionsensitivity Rlift. In this case, since the basic lift correction valueDlift_bs is a value for causing the modified error Weaf to converge to0, correction of the valve lift Liftin using the lift correction valueDlift corresponds to correcting or modifying the valve lift Liftin suchthat the lift error is eliminated. Therefore, calculating the basicestimated intake air amount Gcyl_vt_base by searching the aforementionedFIG. 11 map according to the corrected valve lift Liftin_mod thuscalculated corresponds to calculating the first estimated intake airamount Gcyl_vt as the first input value by using a map modified suchthat the lift error is eliminated.

It should be noted that in the present embodiment, the FIG. 11 mapcorresponds to a correlation model, and the calculation of the firstestimated intake air amount Gcyl_vt by searching the FIG. 11 mapaccording to the corrected valve lift Liftin_mod obtained by correctingthe valve lift Liftin by the lift correction value Dlift corresponds tocalculating the first input value using a modified correlation model.

Next, a control process carried out by the ECU 2 at the above-describedcontrol period ΔTn will be described with reference to FIG. 23. Itshould be noted that various calculated values referred to in thefollowing description are assumed to be stored in the RAM of the ECU 2.

In this process, first, in a step 1 (shown as S1 in abbreviated form inFIG. 23; the following steps are also shown in abbreviated form), thecounter value C_TDC of a TDC counter is set to the sum (C_TDCZ+1) of theimmediately preceding value C_TDCZ of the counter value C_TDC and 1.This means that the counter value C_TDC of the TDC counter isincremented by 1.

Then, the process proceeds to a step 2, wherein it is determined whetheror not C_TDC=4 holds. If the answer to this question is negative (NO),i.e. if C_TDC≠4 holds, the process proceeds to a step 6, describedhereinafter. On the other hand, if the answer to this question isaffirmative (YES), the process proceeds to a step 3, wherein the countervalue C_TDC of the TDC counter is reset to 0.

In a step 4 following the step 3, the target air-fuel ratio KCMD iscalculated. More specifically, as described above, the target air-fuelratio KCMD is calculated by searching the map shown in FIG. 14 accordingto the calculated intake air amount Gcyl and the accelerator pedalopening AP.

Then, in a step 5, the air-fuel ratio correction coefficient KAF iscalculated. More specifically, the air-fuel ratio correction coefficientKAF is calculated with the control algorithm expressed by theaforementioned equations (6) to (10) if conditions for executingair-fuel ratio feedback control are satisfied. On the other hand, if theconditions for executing air-fuel ratio feedback control are notsatisfied, the air-fuel ratio correction coefficient KAF is set to 1.

In the step 6 following the step 2 or 5, an air-fuel ratio controlprocess is executed. The air-fuel ratio control process is provided forcalculating the fuel injection amount TOUT for each fuel injection valve10, and detailed description thereof will be given hereinafter.

Subsequently, in a step 7, an ignition timing control process isperformed. In this process, the ignition timing Iglog is calculated bythe same method as employed in the ignition timing control processdisclosed in Japanese Laid-Open Patent Publication (Kokai) No.2005-315161 referred to hereinabove, though detailed description thereofis omitted here. After that, the present process is terminated.

As described above, in the FIG. 23 control process, the steps 3 to 5 arecarried out whenever C_TDC=4 holds, and hence they are carried outwhenever the total of four successive pulses of the TDC signal aregenerated, i.e. every combustion cycle.

Next, the aforementioned air-fuel ratio control process will bedescribed with reference to FIG. 24. As will be described hereinafter,the present process is for calculating the fuel injection amount TOUTfor each fuel injection valve 10. More specifically, the present processis for calculating the fuel injection amounts TOUT for the fuelinjection valves 10 for the respective cylinders in the order of a firstcylinder→a third cylinder→a fourth cylinder→a second cylinder, as thecounter value C_TDC of the TDC counter is incremented from 1 to 4.

First, in a step 20, the aforementioned corrected valve lift Liftin_mod,air-fuel ratio correction coefficient KAF, and various parameters areread in. In this case, the corrected valve lift Liftin_mod is calculatedat the control period ΔTn, as described above, and hence the reading ofthe corrected valve lift Liftin_mod corresponds to the downsampling ofthe same. Further, since the air-fuel ratio correction coefficient KAFis calculated every combustion cycle, the reading of the air-fuel ratiocorrection coefficient KAF corresponds to the oversampling of the same.

Then, in a step 21, the basic fuel injection amount Tcyl_bs iscalculated. The process for calculating the basic fuel injection amountTcyl_bs is performed as shown in FIG. 25. More specifically, first, in astep 30, the second estimated intake air amount Gcyl_afm is calculatedby the aforementioned equation (3).

Then, in a step 31, as described heretofore, the basic estimated intakeair amount Gcyl_vt_base is calculated by searching the FIG. 11 mapaccording to the engine speed NE and the corrected valve liftLiftin_mod.

In a step 32 following the step 31, as described heretofore, thecorrection coefficient K_gcyl_vt is calculated by searching the FIG. 12map according to the engine speed NE and the cam phase Cain.

After that, the process proceeds to a step 33, wherein the firstestimated intake air amount Gcyl_vt is calculated by the aforementionedequation (1) based on the two values Gcyl_vt_base and K_gcyl_vtcalculated in the steps 31 and 32.

Next, in a step 34, the estimated flow rate Gin_vt is calculated by theaforementioned equation (2), and thereafter the process proceeds to astep 35, wherein it is determined whether or not a variable mechanismfailure flag F_VDNG is equal to 1.

The variable mechanism failure flag F_VDNG is set to 1 when it isdetermined in a failure determination process, not shown, that at leastone of the variable valve lift mechanism 50 and the variable cam phasemechanism 70 is faulty, and to 0 when it is determined that themechanisms 50 and 70 are both normal. It should be noted that in thefollowing description, the variable valve lift mechanism 50 and thevariable cam phase mechanism 70 are collectively referred to as “the twovariable mechanisms”.

If the answer to the question of the step 35 is negative (NO), i.e. ifboth of the two variable mechanisms are normal, the process proceeds toa step 36, wherein it is determined whether or not an air flow sensorfailure flag F_AFMNG is equal to 1. The air flow sensor failure flagF_AFMNG is set to 1 when it is determined in a failure determinationprocess, not shown, that the air flow sensor 22 is faulty, and to 0 whenit is determined that the air flow sensor 22 is normal.

If the answer to the question of the step 36 is negative (NO), i.e. ifthe air flow sensor 22 is normal, the process proceeds to a step 37,wherein as described above, the transition coefficient Kg is calculatedby searching the FIG. 13 map according to the estimated flow rateGin_vt.

On the other hand, if the answer to the question of the step 36 isaffirmative (YES), i.e. if the air flow sensor 22 is faulty, the processproceeds to a step 38, wherein the transition coefficient Kg is set to0.

In a step 39 following the step 37 or 38, the calculated intake airamount Gcyl is calculated by the aforementioned equation (4). Then, in astep 40, the basic fuel injection amount Tcyl_bs is set to the productKgt Gcyl of the conversion coefficient and the calculated intake airamount Gcyl, followed by terminating the present process.

On the other hand, if the answer to the question of the step 35 isaffirmative (YES), i.e. if it is determined that at least one of the twovariable mechanisms is faulty, the process proceeds to a step 41,wherein the calculated intake air amount Gcyl is set to theaforementioned predetermined failure-time value Gcyl_fs. Then, theaforementioned step 40 is executed, followed by terminating the presentprocess.

Referring again to FIG. 24, in the step 21, the basic fuel injectionamount Tcyl_bs is calculated, as described above, and then the processproceeds to a step 22, wherein the total correction coefficient KTOTALis calculated. More specifically, as described above, the totalcorrection coefficient KTOTAL is calculated by calculating variouscorrection coefficients by searching respective associated mapsaccording to operating parameters (e.g. the intake air temperature TA,the atmospheric pressure PA, the engine coolant temperature TW, theaccelerator pedal opening AP, etc.), and then multiplying the thuscalculated correction coefficients by each other.

Next, the process proceeds to a step 23, wherein the demanded fuelinjection amount Tcyl is calculated by the aforementioned equation (11).Then, in a step 24, the fuel injection amount TOUT is calculated bycarrying out a predetermined fuel attachment-dependent correctionprocess on the demanded fuel injection amount Tcyl, as described above,followed by terminating the present process. Thus, each fuel injectionvalve 10 is controlled such that the fuel injection timing and thevalve-opening time period thereof assume values determined based on thefuel injection amount TOUT. As a result, if the conditions for executingthe air-fuel ratio feedback control are satisfied, the actual air-fuelratio KACT is controlled such that it converges to the target air-fuelratio KCMD.

Next, a control process executed by the ECU 2 at the control period ΔTkset by a timer will be described with reference to FIG. 26. In thisprocess, first, in a step 50, data stored in the RAM, such as the firstestimated intake air amount Gcyl_vt, the second estimated intake airamount Gcyl_afm, the actual air-fuel ratio KACT, and the air-fuel ratiocorrection coefficient KAF, are read in.

Then, the process proceeds to a step 51, wherein it is determinedwhether or not a feedback control execution flag F_AFFB is equal to 1.The feedback control execution flag F_AFFB is set to 1 during executionof the air-fuel ratio feedback control, and otherwise to 0.

If the answer to the question of the step 51 is affirmative (YES), i.e.if the air-fuel ratio feedback control is being executed, the processproceeds to a step 52, wherein it is determined whether or not theengine coolant temperature TW is higher than a predetermined referencevalue TWREF. The predetermined reference value TWREF is a value fordetermining whether or not the warmup operation of the engine 3 has beenterminated.

If the answer to the question of the step 52 is affirmative (YES), i.e.if the warmup operation of the engine 3 has been terminated, the processproceeds to a step 53, wherein it is determined whether or not a purgecompletion flag F_CANI is equal to 1. The purge completion flag F_CANIis set to 1 when a purge operation for returning evaporated fueladsorbed by a canister into a intake passage has been completed, andotherwise to 0.

If the answer to the question of the step 53 is affirmative (YES), i.e.if the purge operation has been completed, the process proceeds to astep 54, wherein a process for calculating the corrected valve liftLiftin_mod is carried out. The process for calculating the correctedvalve lift Liftin_mod will be described in detail hereinafter.

On the other hand, if any of the answers to the questions of the steps51 to 53 is negative (NO), it is judged that conditions for calculatingthe corrected valve lift Liftin_mod are not satisfied, and the processproceeds to a step 56, wherein the corrected valve lift Liftin_mod isset to the immediately preceding value Liftin_modz thereof. As describedabove, if the air-fuel ratio feedback control is not being executed, ifthe warmup operation of the engine 3 has not been terminated, or if thepurge operation has not been completed, the air-fuel ratio controlbecomes unstable, and the calculation accuracy of the lift correctionvalue Dlift is lowered, which can lower the calculation accuracy of thecorrected valve lift Liftin_mod. To avoid this problem, the immediatelypreceding value of the corrected valve lift Liftin_mod is used withoutupdating the corrected valve lift Liftin_mod.

In a step 55 following the step 54 or 56, a variable mechanism controlprocess is performed, as described hereinafter, followed by terminatingthe present process.

Next, the above-described process for calculating the corrected valvelift Liftin_mod will be described with reference to FIG. 27. First, in astep 60, the air-fuel ratio error estimated value Eaf is calculated bythe aforementioned equations (12) and (13).

Then, the process proceeds to a step 61, wherein the second correctedvalve lift Liftin_mod_p is calculated by the aforementioned equation(14). After that, in a step 62, the basic error weight W_base iscalculated by searching the FIG. 19 map according to the secondcorrected valve lift Liftin_mod_p and the engine speed NE.

In a step 63 following the step 62, the error weight correctioncoefficient K_w is calculated by searching the aforementioned FIG. 20map according to the cam phase Cain and the engine speed NE.

Next, in a step 64, the error weight W is calculated by theaforementioned equation (15), whereafter the process proceeds to a step65, wherein the modified error Weaf is calculated by the aforementionedequation (16).

In a step 66 following the step 65, the basic lift correction valueDlift_bs is calculated by the aforementioned equations (17) to (24), andthen the process proceeds to a step 67, wherein the basic sensitivityR_base is calculated by searching the aforementioned FIG. 21 mapaccording to the second corrected valve lift Liftin_mod_p and the enginespeed NE.

Then, the process proceeds to a step 68, wherein the sensitivitycorrection coefficient K_r is calculated by searching the aforementionedFIG. 22 map according to the cam phase Cain and the engine speed NE.After that, in a step 69, the correction sensitivity Rlift is calculatedby the aforementioned equation (25).

In a step 70 following the step 69, the lift correction value Dlift iscalculated by the aforementioned equation (26). Next, the processproceeds to a step 71, wherein the corrected valve lift Liftin_mod iscalculated by the aforementioned equation (27), followed by terminatingthe present process.

Next, the aforementioned variable mechanism control process will bedescribed with reference to FIG. 28. The present process is forcalculating the two control inputs U_Liftin and U_Cain for controllingthe two variable mechanisms, respectively.

In this process, first, it is determined in a step 80 whether or not theaforementioned variable mechanism failure flag F_VDNG is equal to 1. Ifthe answer to this question is negative (NO), i.e. if the two variablemechanisms are both normal, the process proceeds to a step 81, whereinit is determined whether or not the engine start flag F_ENGSTART isequal to 1.

The above engine start flag F_ENGSTART is set by determining in adetermination process, not shown, whether or not engine start control isbeing executed, i.e. the engine 3 is being cranked, based on the enginespeed NE and the ON/OFF signal output from an IG·SW 29. Morespecifically, when the engine start control is being executed, theengine start flag F_ENGSTART is set to 1, and otherwise set to 0.

If the answer to the question of the step 81 is affirmative (YES), i.e.if the engine start control is being executed, the process proceeds to astep 82, wherein the target valve lift Liftin_cmd is calculated bysearching a map shown in FIG. 29 according to the engine coolanttemperature TW.

In this map, in the range where the engine coolant temperature TW ishigher than a predetermined value TWREF1, the target valve liftLiftin_cmd is set to a larger value as the engine coolant temperature TWis lower, and in the range where TW≦TWREF1 holds, the target valve liftLiftin_cmd is set to a predetermined value Liftinref. This is tocompensate for an increase in friction of the variable valve liftmechanism 50, which is caused when the engine coolant temperature TW islow.

Then, in a step 83, the target cam phase Cain_cmd is calculated bysearching a map shown in FIG. 30 according to the engine coolanttemperature TW.

In this map, in the range where the engine coolant temperature TW ishigher than a predetermined value TWREF2, the target cam phase Cain_cmdis set to a more retarded value as the engine coolant temperature TW islower, and in the range where TW≦TWREF2 holds, the target cam phaseCain_cmd is set to a predetermined value Cainref. This is to ensure thecombustion stability of the engine 3 by controlling the cam phase Cainto a more retarded value when the engine coolant temperature TW is lowthan when the engine coolant temperature TW is high, to thereby reducethe valve overlap, to increase the flow velocity of intake air.

Next, the process proceeds to a step 84, wherein the lift control inputU_Liftin is calculated with a target value filter-typetwo-degree-of-freedom response-specifying control algorithm expressed bythe following equations (28) to (31).

$\begin{matrix}{{{{U\_ Liftin}(k)} = {{{{- {Krch\_ lf}} \cdot {\sigma\_ lf}}(k)} - {{Kadp\_ lf} \cdot {\sum\limits_{i = 0}^{k}{{\sigma\_ lf}(i)}}}}}\;} & (28) \\{{{\sigma\_ lf}(k)} = {{{E\_ lf}(k)} + {{{pole\_ lf} \cdot {E\_ lf}}( {k - 1} )}}} & (29) \\{{{E\_ lf}(k)} = {{{Liftin\_ mod}(k)} - {{Liftin\_ cmd}{\_ f}(k)}}} & (30) \\{{{Liftin\_ cmd}{\_ f}(k)} = {{{- {pole\_ f}}{{\_ lf} \cdot {Liftin\_ cmd}}{\_ f}( {k - 1} )} + {{( {l + {{pole\_ f}{\_ lf}}} ) \cdot {Liftin\_ cmd}}(k)}}} & (31)\end{matrix}$

In the equation (28), Krch_lf and Kadp_lf represent a predeterminedreaching law gain and a predetermined adaptive law gain, respectively.Furthermore, σ_lf represents a switching function defined by theequation (29). In the equation (29), pole_lf represents a switchingfunction-setting parameter set to a value which satisfies therelationship of −1<pole_lf<0, and E_lf represents a follow-up errorcalculated by the equation (30). In the equation (30), Liftin_cmd_frepresents a filtered value of the target valve lift, and is calculatedwith a first-order lag filter algorithm expressed by the equation (31).In the equation (31), pole_f_lf represents a target value filter-settingparameter set to a value which satisfies the relationship of−1<pole_f_lf<0.

Next, the process proceeds to a step 85, wherein the phase control inputU_Cain is calculated with a target value filter-typetwo-degree-of-freedom response-specifying control algorithm expressed bythe following equations (32) to (35).

$\begin{matrix}{{{U\_ Cain}(k)} = {{{{- {Krch\_ ca}} \cdot {\sigma\_ ca}}(k)} - {{Kadp\_ ca} \cdot {\sum\limits_{i = 0}^{k}{{\sigma\_ ca}(i)}}}}} & (32) \\{{{\sigma\_ ca}(k)} = {{{E\_ ca}(k)} + {{{pole\_ ca} \cdot {E\_ ca}}( {k - 1} )}}} & (33) \\{{{E\_ ca}(k)} = {{{Cain}(k)} - {{Cain\_ cmd}{\_ f}(k)}}} & (34) \\{{{Cain\_ cmd}{\_ f}(k)} = {{{- {pole\_ f}}{{\_ ca} \cdot {Cain\_ cmd}}{\_ f}( {k - 1} )} + {{( {1 + {{pole\_ f}{\_ ca}}} ) \cdot {Cain\_ cmd}}(k)}}} & (35)\end{matrix}$

In the equation (32), Krch_ca and Kadp_ca represent a predeterminedreaching law gain and a predetermined adaptive law gain, respectively.Furthermore, σ_ca represents a switching function defined by theequation (33). In the equation (33), pole_ca represents a switchingfunction-setting parameter set to a value which satisfies therelationship of −1<pole_ca<0, and E_ca represents a follow-up errorcalculated by the equation (34). In the equation (34), Cain_cmd_frepresents a filtered value of the target cam phase, and is calculatedwith a first-order lag filter algorithm expressed by the equation (35).In the equation (35), pole_f_ca represents a target value filter-settingparameter set to a value which satisfies the relationship of−1<pole_f_ca<0.

In the step 85, the phase control input U_Cain is calculated as above,followed by terminating the present process.

On the other hand, if the answer to the question of the step 81 isnegative (NO), i.e. if the engine start control is not being executed,the process proceeds to a step 86, wherein it is determined whether ornot the accelerator pedal opening AP is smaller than a predeterminedvalue APREF. If the answer to this question is affirmative (YES), i.e.if the accelerator pedal is not stepped on, the process proceeds to astep 87, wherein it is determined whether or not the count Tast of anafter-start timer is smaller than a predetermined value Tastlmt.

If the answer to this question is affirmative (YES), i.e. ifTast<Tastlmt holds, it is judged that the catalyst warmup control shouldbe executed, and the process proceeds to a step 88, wherein the targetvalve lift Liftin_cmd is calculated by searching a map shown in FIG. 31according to the count Tast of the after-start timer and the enginecoolant temperature TW. In FIG. 31, TW1 to TW3 represent predeterminedvalues of the engine coolant temperature TW, which satisfy therelationship of TW1<TW2<TW3. This also applies to the followingdescription.

In this map, the target valve lift Liftin_cmd is set to a larger valueas the engine coolant temperature TW is lower. This is because as theengine coolant temperature TW is lower, it takes a longer time period toactivate the catalyst, and hence the volume of exhaust gasses isincreased to shorten the time period required for activating thecatalyst. Furthermore, in the above map, the target valve liftLiftin_cmd is set to a larger value as the count Tast of the after-starttimer becomes larger in the range where the count Tast is small, whereasin a region where the count Tast is large to a certain or more extent,the target valve lift Liftin_cmd is set to a smaller value as the countTast becomes larger. This is because the warming up of the engine 3proceeds along with the lapse of the execution time period of thecatalyst warmup control, so that after friction lowers, unless theintake air amount is reduced, the ignition timing is excessivelyretarded so as to hold the engine speed NE at the target value, whichmakes unstable the combustion state of the engine. To avoid thecombustion state from being unstable, the map is configured as describedabove.

Then, in a step 89, the target cam phase Cain_cmd is calculated bysearching a map shown in FIG. 32 according to the count Tast of theafter-start timer and the engine coolant temperature TW.

In this map, the target cam phase Cain_cmd is set to a more advancedvalue as the engine coolant temperature TW is lower. This is because asthe engine coolant temperature TW is lower, it takes a longer timeperiod to activate the catalyst, as described above, and hence thepumping loss is reduced to increase the intake air amount to therebyshorten the time period required for activating the catalyst.Furthermore, in the above map, the target cam phase Cain_cmd is set to amore retarded value as the count Tast of the after-start timer becomeslarger in the range where the count Tast of the after-start timer issmall, whereas in a region where the count Tast is large to a certain ormore extent, the target cam phase Cain_cmd is set to a more advancedvalue as the count Tast of the after-start timer is larger. The reasonfor this is the same as given in the description of the FIG. 31 map.

Then, the steps 84 and 85 are carried out, as described hereinabove,followed by terminating the present process.

On the other hand, if the answer to the question of the step 86 or 87 isnegative (NO), i.e. if the accelerator pedal is stepped on, or ifTast≧Tastlmt holds, the process proceeds to a step 90, wherein thetarget valve lift Liftin_cmd is calculated by searching a map shown inFIG. 33 according to the engine speed NE and the accelerator pedalopening AP. In FIG. 33, AP1 to AP3 indicate predetermined values of theaccelerator pedal opening AP which satisfy the relationship ofAP1<AP2<AP3. This also applies to the following description.

In this map, the target valve lift Liftin_cmd is set to a larger valueas the engine speed NE is higher, or as the accelerator pedal opening APis larger. This is because as the engine speed NE is higher, or as theaccelerator pedal opening AP is larger, an output required of the engine3 is larger, and hence a larger intake air amount is required.

Then, in a step 91, the target cam phase Cain_cmd is calculated bysearching a map shown in FIG. 34 according to the engine speed NE andthe accelerator pedal opening AP. In this map, when the acceleratorpedal opening AP is small and the engine speed NE is in the medium speedregion, the target cam phase Cain_cmd is set to a more advanced valuethan otherwise. This is because under the above operating conditions ofthe engine 3, it is necessary to reduce the pumping loss.

Following the step 91, the steps 84 and 85 are carried out, as describedhereinabove, followed by terminating the present process.

On the other hand, if the answer to the question of the step 80 isaffirmative (YES), i.e. if at least one of the two variable mechanismsis faulty, the process proceeds to a step 92, wherein the lift controlinput U_Liftin is set to the predetermined failure time valueU_Liftin_fs, and the phase control input U_Cain to the predeterminedfailure time value U_Cain_fs, followed by terminating the presentprocess. As a result, as described above, the valve lift Liftin is heldat the predetermined locked value, and the cam phase Cain at thepredetermined locked value, whereby it is possible to suitably carry outidling or starting of the engine 3 during stoppage of the vehicle, andat the same time hold the vehicle in the state of low-speed travelingwhen the vehicle is traveling.

In the present process, the lift control input U_liftin and the phasecontrol input U_Cain are calculated as described above. Then, byinputting these control inputs U_Liftin and U_Cain to the variable valvelift mechanism 50 and the variable cam phase mechanism 70, respectively,the intake air amount is controlled.

Next, a description will be given of the results of control by thecontrol apparatus 1 according to the first embodiment configured asdescribed above. FIG. 35 shows an example of the results of the air-fuelratio control and the variable mechanism control process carried out bythe control apparatus 1 according to the present embodiment.

As shown in FIG. 35, during execution of the air-fuel ratio controlusing the transition coefficient Kg set to 0, based on the firstestimated intake air amount Gcyl_vt alone, if a sudden increase of theair-fuel ratio error estimated value Eaf in a positive direction iscaused e.g. by the lift error at a time point t1, the modified errorWeaf as well is suddenly increased simultaneously. As a result, thebasic lift correction value Dlift_bs is changed in a negative directionwith the control algorithm expressed by the aforementioned equations(17) to (24) such that the modified error Weaf is caused to converge to0, suddenly increasing the absolute value of the basic lift correctionvalue Dlift_bs. In short, the basic lift correction value Dlift_bs iscalculated such that the lift error is eliminated.

In addition, the basic lift correction value Dlift_bs becomes not largerthan the aforementioned predetermined lower limit value Dlift_bs_L atthe time point t1, whereby the forgetting coefficient λ is switched from1 to the predetermined value λlmt. Then, after the time point t1, theabsolute value of the basic lift correction value Dlift_bs is reducedalong with the lapse of time due to the forgetting effect provided bythe forgetting coefficient λ, and when Dlift_bs>Dlift_bs_L comes to hold(time point t2), the forgetting coefficient λ is switched from thepredetermined value λlmt to 1. As a result, the forgetting effectprovided by the forgetting coefficient λ is cancelled, and the adaptivelaw input Uadp is calculated as an integral value of a switchingfunction a by the aforementioned equations (20) and (21), whereby thebasic lift correction value Dlift_bs is calculated as a value capable ofeliminating the lift error quickly and properly, by the function of theadaptive law input Uadp.

Then, after a time point t3, when load on the engine 3 is changed toprogressively increase the transition coefficient Kg from 0, themodified error Weaf is reduced along with the progressive increase inthe transition coefficient Kg, and when the transition coefficient Kg=1comes to hold (time point t4), the modified error Weaf becomes equal to0, but by the function of the adaptive law input Uadp, the basic liftcorrection value Dlift_bs is held at the value capable of eliminatingthe lift error quickly and properly without converging to 0, even afterthe time point t4.

Thereafter, at a time point t5, when the transition coefficient Kgstarts to decrease progressively from 1, the modified error Weafincreases in a positive direction along with the decrease in thetransition coefficient Kg, and the basic lift correction value Dlift_bschanges on the negative side such that the absolute value thereofincreases. Consequently, it is understood that the lift error isproperly compensated for after the first estimated intake air amountGcyl_vt has started to be reflected on the calculation of the fuelinjection amount TOUT. Then, the basic lift correction value Dlift_bs iscalculated such that after a time point t6, the transition coefficientKg is held at a positive value smaller than 1, and the modified errorWeaf converges to 0.

Further, FIG. 36 shows an example of the results of the air-fuel ratiocontrol carried out by the control apparatus 1 according to the firstembodiment. For comparison, FIG. 37 shows an example (hereinafterreferred to as “the comparative example”) of control results obtainedwhen the lift correction value Dlift is held at 0, i.e. when Liftin_modis set to be equal to Liftin. It should be noted that the above controlresults are obtained by setting the target air-fuel ratio KCMD to 1 forease of understanding.

Referring to FIGS. 36 and 37, it is understood that in the FIG. 37comparative example, there often occurs a state in which the air-fuelratio correction coefficient KAF largely deviates from the targetair-fuel ratio KCMD toward the richer side, and is held on the richerside. In contrast, it is understood that in the FIG. 36 example of thecontrol results by the control apparatus 1 according to the presentembodiment, the air-fuel ratio correction coefficient KAF is held in thevicinity of the target air-fuel ratio KCMD and that high-level controlaccuracy can be ensured.

Further, when the differences between the target air-fuel ratios KCMDand the actual air-fuel ratios KACT, that is, errors of the air-fuelratios of the example and the comparative example are compared with eachother by referring to FIGS. 36 and 37, it is understood that relativelylarge air-fuel ratio errors occur frequently in the comparative example.In contrast, it is understood that in the example of the control resultsaccording to the present embodiment, air-fuel ratio errors arecontrolled to smaller values than the values of the air-fuel ratioerrors in the comparative example, whereby high-level control accuracycan be secured. As described above, it is understood that by using thelift correction value Dlift according to the present embodiment, it ispossible to accurately compensate for the lift error, thereby making itpossible to ensure high control accuracy in the air-fuel ratio control.

As described hereinabove, according to the control apparatus 1 of thefirst embodiment, the error weight W is calculated using a responsesurface model configured as shown in FIGS. 19 and 20, and the air-fuelratio error estimated value Eaf is corrected (modified) by the errorweight W, to thereby calculate the air-fuel ratio error estimated valueEaf. As described hereinabove, the error weight W is calculated as avalue indicative of the probability of the air-fuel ratio errorestimated value Eaf being caused by the lift error, in other words, thedegree of influence of the valve lift Liftin on the air-fuel ratio errorestimated value Eaf, and therefore the modified error Weaf is calculatedas a value on which is reflected the degree of influence of the valvelift Liftin on the air-fuel ratio error estimated value Eaf.

Further, the basic lift correction value Dlift_bs is calculated suchthat the modified error Weaf calculated as above is caused to convergeto 0, and the lift correction value Dlift is calculated by multiplyingthe basic lift correction value Dlift_bs by the correction sensitivityRlift. The basic estimated intake air amount Gcyl_vt_base, i.e. thefirst estimated intake air amount Gcyl_vt is calculated by searching theFIG. 11 map, i.e. the correlation model, according to the correctedvalve lift Liftin_mod obtained by correcting the valve lift Liftin bythe lift correction value Dlift. Therefore, not only when the air-fuelratio error is temporarily increased by a disturbance but also when theair-fuel ratio error estimated value Eaf, i.e. the air-fuel ratio erroris liable to temporarily increase due to occurrence of the lift errorcaused e.g. by the degradation of reliability of the detection resultsof the valve lift Liftin and a change in the characteristics of thevariable valve lift mechanism 50, the air-fuel ratio error can beproperly compensated for just enough by the first estimated intake airamount Gcyl_vt.

If the first estimated intake air amount Gcyl_vt is calculated assumingthat the error weight=1 and Eaf=Weaf hold, when the air-fuel ratio errorestimated value Eaf is caused mainly by the lift error, i.e. when thedegree of influence of the valve lift Liftin on the air-fuel ratio errorestimated value Eaf is large, the air-fuel ratio error estimated valueEaf, i.e. the air-fuel ratio error can be properly compensated for bythe thus calculated first estimated intake air amount Gcyl_vt. However,when the degree of influence of the valve lift Liftin on the air-fuelratio error estimated value Eaf is small, i.e. when the air-fuel ratioerror is caused mainly by a disturbance or the like other than the lifterror, it is impossible to properly compensate for the air-fuel ratioerror using the first estimated intake air amount Gcyl_vt, which resultsin overcompensation or undercompensation for the air-fuel ratio error.Therefore, by using the above-described error weight W, the air-fuelratio error can be properly compensated for just enough by the firstestimated intake air amount Gcyl_vt.

In addition, since the first estimated intake air amount Gcyl_vt iscalculated using the FIG. 11 map which represents the correlationbetween the corrected valve lift Liftin_mod and the first estimatedintake air amount Gcyl_vt, the air-fuel ratio error can be compensatedfor more quickly than when the air-fuel ratio error is compensated forby the air-fuel ratio correction coefficient KAF calculated with afeedback control algorithm. As described above, even under a conditionwhere the air-fuel ratio error is temporarily increased by the lifterror, it is possible to compensate for the air-fuel ratio errorproperly and quickly, thereby making it possible to ensure high-levelaccuracy of control even when the engine 3 is in a transient operatingstate.

Furthermore, although the sensitivity of the air-fuel ratio errorestimated value Eaf to the lift error varies with the influence of thecam phase Cain and the engine speed NE thereon, the error weight W iscalculated based not only on the valve lift Liftin but also on the camphase Cain and the engine speed NE, as described above, and hence theerror weight W is calculated such that the degree of influence of thecam phase Cain and the engine speed NE on the air-fuel ratio errorestimated value Eaf is reflected. Accordingly, by using the error weightW calculated as above, it is possible to calculate the first estimatedintake air amount Gcyl_vt such that the air-fuel ratio error estimatedvalue Eaf, i.e. the air-fuel ratio error can be compensated for, whilecausing the influence of the cam phase Cain and the engine speed NE onthe lift error to be reflected thereon. This makes it possible tofurther enhance the accuracy of control.

Further, the correction sensitivity Rlift is calculated as a value whichrepresents the sensitivity of the air-fuel ratio error estimated valueEaf to the lift error, and the lift correction value Dlift is calculatedby multiplying the basic lift correction value Dlift_bs by thecorrection sensitivity Rlift, so that it is possible to prevent thebasic lift correction value Dlift_bs from effecting overcompensationresponsive to the air-fuel ratio error estimated value Eaf under thecondition, described above, where the sensitivity of the air-fuel ratioerror estimated value Eaf to the lift error is low. In addition, thevalue of the sensitivity correction coefficient K_r, which is used forcalculating the correction sensitivity Rlift, on the advanced sidethereof is set to a value closer to 1 than that of the error weightcorrection coefficient K_w, which is used for calculating the errorweight W, whereby it is possible to prevent the air-fuel mixture frombeing leaned by the fuel injection amount TOUT erroneously calculated asa small value, as described above. This makes it possible to ensure thestability of combustion.

It should be noted that although in the first embodiment, the controlalgorithm expressed by the aforementioned equations (17) to (24) is usedfor the algorithm for calculating the basic lift correction valueDlift_bs, by way of example, this is not limitative, but the basic liftcorrection value Dlift_bs may be calculated with a control algorithmexpressed by the following equations (36) to (44), to which are applieda combination of an adaptive disturbance observer and a sliding modecontrol algorithm.

$\begin{matrix}{{\sigma(k)} = {{{Weaf}(k)} + {S \cdot {{Weaf}( {k - 1} )}}}} & (36) \\{{{Urch}(k)} = {{- {Krch}} \cdot {\sigma(k)}}} & (37) \\{{{Unl}(k)} = {{- {Knl}} \cdot {{sgn}( {\sigma(k)} )}}} & (38) \\{{{\sigma\_ hat}(k)} = {{{Urch}( {k - 1} )} + {{Unl}( {k - 1} )} + {{Uls}( {k - 1} )}}} & (39) \\\begin{matrix}{{{E\_ sig}(k)} = {{\sigma(k)} - {\sigma\_ hat}}} \\{= {{\sigma(k)} - {{Urch}( {k - 1} )} - {{Unl}( {k - 1} )} - {{Uls}( {k - 1} )}}}\end{matrix} & (40) \\{{{Uls}(k)} = {{\lambda \cdot {{Uls}( {k - 1} )}} + {\frac{P}{1 + P}{E\_ sig}(k)}}} & (41) \\{{{*{When}\mspace{14mu}{Dlift\_ bs}{\_ L}} < {{Dlift\_ bs}( {k - 1} )} < {{Dlift\_ bs}{\_ H}}}{\lambda = 1}} & (42) \\{{{*{When}\mspace{14mu}{Dlift\_ bs}( {k - 1} )} \leqq {{Dlift\_ bs}{\_ L}\mspace{14mu}{or}\mspace{14mu}{Dlift\_ bs}{\_ H}} \leqq {{Dlift\_ bs}( {k - 1} )}}{\lambda = {\lambda\;{lmt}}}} & (43) \\{{{Dlift\_ bs}(k)} = {{{Urch}(k)} + {{Unl}(k)} + {{Uls}(k)}}} & (44)\end{matrix}$

In the above equation (39), σ_hat represents an estimated value of aswitching function, and Uls represents a disturbance estimated value.The disturbance estimated value Uls is calculated with a fixed gainidentification algorithm expressed by the equations (40) and (41). Inthe equation (40), E_sig represents an estimation error. In the equation(41), P represents a fixed identification gain. It should be noted thatthe above equations (39) to (43) express an algorithm for calculatingthe disturbance estimated value Uls of the adaptive disturbanceobserver.

In the above control algorithm expressed by the equations (36) to (44)for calculation of the basic lift correction value Dlift_bs, thedisturbance estimated value Uls corresponds to an integral term. In theequation (41), the immediately preceding value Uls(k−1) of thedisturbance estimated value is multiplied by the forgetting coefficientλ, and if the absolute value of the basic lift correction value Dlift_bsis large, the forgetting coefficient λ is set to a value within therange of 0≦λ≦1. With this configuration, the aforementioned forgettingeffect provided by the forgetting coefficient λ makes it possible toprevent the integral terms Uadp′ and Uls in the respective controlalgorithms for calculating the air-fuel ratio correction coefficient KAFand the basic lift correction value Dlift_bs from interfering with eachother to thereby prevent the integral terms from exhibiting oscillatingbehaviors, and the absolute value of the disturbance estimated valueUls, i.e. the basic lift correction value Dlift_bs from increasing. Thismakes it possible to prevent the first estimated intake air amountGcyl_vt from oscillating and temporarily taking an improper value,thereby making it possible to improve controllability in a transientstate. Further, if the absolute value of the immediately preceding valueDlift_bs(k−1) of the basic lift correction value is small, theforgetting coefficient λ is set to 1, and hence even when the modifiederror Weaf become close to 0, the basic lift correction value Dlift_bscan be held at a proper value. This makes it possible to enhance theresponsiveness of the air-fuel ratio control when the modified errorWeaf start to increase, thereby making it possible to enhance thecontrol accuracy.

In addition, since the disturbance estimated value Uls is calculatedwith the fixed gain identification algorithm of the adaptive disturbanceobserver, compared with the control algorithm according to the firstembodiment using the adaptive law input Uadp, it is possible to furtherenhance the capability of suppressing the integral fluctuation behaviorand the overshooting behavior of the basic lift correction valueDlift_bs.

Further, although in the first embodiment, the basic lift correctionvalue Dlift_bs is calculated using the control algorithm to which isapplied the sliding mode control algorithm expressed by the equations(17) to (24) as the response-specifying control algorithm, by way ofexample, a control algorithm to which is applied a back-stepping controlalgorithm may be used as the response-specifying control algorithm. Alsowhen the control algorithm to which is applied the back-stepping controlalgorithm is used for the algorithm for calculating the basic liftcorrection value Dlift_bs, as described above, it is possible to obtainthe same advantageous effects as provided by the control algorithmexpressed by the equations (17) to (24) in the first embodiment.

Further, although in the first embodiment, the control algorithmexpressed by the aforementioned equations (17) to (24) is used for thealgorithm for calculating the basic lift correction value Dlift_bs, byway of example, the algorithm for calculating the basic lift correctionvalue Dlift_bs is not limited to this, but any suitable algorithm may beused insofar as it is capable of calculating the basic lift correctionvalue Dlift_bs such that the modified error Weaf is caused to convergeto 0. For example, a PID control algorithm, an optimum controlalgorithm, an H^(∞) control algorithm, or the like may be used for thealgorithm for calculating the basic lift correction value Dlift_bs. Whenthe basic lift correction value Dlift_bs is thus calculated with the PIDcontrol algorithm, the optimum control algorithm, the H^(∞) controlalgorithm, or the like, compared with the use of the control algorithmexpressed by the equations (17) to (24), there is a fear that the effectof suppressing the modified error Weaf from overshooting 0, or therobustness of the control apparatus from being degraded, and hence inthis respect, the control algorithm according to the first embodiment issuperior to the PID control algorithm, the optimum control algorithm,the H^(∞) control algorithm, and so forth.

Further, although in the first embodiment, the control algorithmexpressed by the aforementioned equations (6) to (10) is used for apredetermined feedback control algorithm for calculating the air-fuelratio correction coefficient KAF as the second input value, by way ofexample, the predetermined feedback control algorithm for calculatingthe second input value in the present invention is not limited to this,but any suitable feedback control algorithm may be used insofar as it iscapable of calculating the second input value such that the second inputvalue is caused to converge to the target value. For example, theair-fuel ratio correction coefficient KAF as the second input value maybe calculated with an algorithm using a self tuning regulator, which isdisclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No.2006-2591. Further, as the algorithm for calculating the air-fuel ratiocorrection coefficient KAF as the second input value, there may be usedthe control algorithm expressed by the aforementioned equations (36) to(44), or may be used the back-stepping control algorithm, the PIDcontrol algorithm, the optimum control algorithm, the H^(∞) controlalgorithm, or the like.

Furthermore, although in the first embodiment, the correctionsensitivity Rlift is calculated using the response surface model formedby the maps shown in FIGS. 21 and 22, by way of example, the correctionsensitivity Rlift may be calculated using the response surface modelformed by the maps shown in FIGS. 19 and 20 in place of the responsesurface model formed by the maps shown in FIGS. 21 and 22. In short, thecorrection sensitivity Rlift may be calculated as a value equal to theerror weight W. Furthermore, if there is no need to avoid theovercompensation for the air-fuel ratio error estimated value Eaf by thelift correction value Dlift under the condition where the sensitivity ofthe air-fuel ratio error estimated value Eaf to the lift error is low,the equation (25) may be omitted to set Rlift to 1 in the equation (26)to thereby to cause Dlift=Dlift_bs to hold. That is, the basic liftcorrection value Dlift_bs may be used as the lift correction valueDlift.

Further, although in the first embodiment, the valve lift Liftin is usedas an operating state parameter, by way of example, the operating stateparameter in the control apparatus according to the present invention isnot limited to this. For example, to control the air-fuel ratio of theengine 3 having the variable cam phase mechanism 70, the cam phase Cainmay be used as an operating state parameter. Further, to control theair-fuel ratio of the engine 3, which is not provided with the variablevalve lift mechanism 50 or the variable cam phase mechanism 70 but witha throttle valve mechanism alone, the degree of opening of the throttlevalve mechanism may be used as an operating state parameter. Inaddition, in the case of the so-called speed-density engine, which isprovided with an intake pipe pressure sensor and a crank angle sensor,for controlling the air-fuel ratio based on parameters from the sensors,the intake pipe pressure and the engine speed NE may be used asoperating state parameters.

Furthermore, although in the first embodiment, to modify the correlationmodel, the valve lift Liftin as the second reference parameter iscorrected (modified) by the lift correction value Dlift as the correctedmodification value, by way of example, the method for modifying thecorrelation model according to the present invention is not limited tothis, but any suitable method may be used insofar as it is a methodcapable of modifying the correlation model. For example, a method may beemployed in which the first input value is modified using the correctedmodification value.

Next, a control apparatus 1A (see FIG. 38) according to a secondembodiment of the present invention will be described. It should benoted that in the following description, component elements of thecontrol apparatus 1A, identical to those of the control apparatus 1according to the first embodiment, are designated by identical referencenumerals, and detailed description thereof is omitted. The controlapparatus 1A is applied to a vehicle of a so-called FR system, notshown, which has the engine 3 with the aforementioned automatictransmission installed on a front side thereof, and includes rear wheelsand front wheels, neither of which is shown, as drive wheels andnon-drive wheels, respectively. More specifically, the control apparatus1A is provided for carrying out traction control of the vehicle.

It should be noted that the term “traction control” is intended to meana control method of reducing engine torque, when the engine torquebecomes too large during acceleration of the vehicle, thereby causing astate in which the drive wheels rotate without load or idle with respectto the non-drive wheels, so as to avoid the idling state to therebyensure the stability of the vehicle to enhance the accelerationperformance of the engine 3.

Referring to FIG. 38, the control apparatus 1A includes the ECU 2. Tothe ECU 2 are connected not only the aforementioned sensors 20 to 27 butalso left and right front wheel speed sensors 80 and 81, and left andright rear wheel speed sensors 82 and 83. It should be noted that in thepresent embodiment, the crank angle sensor 20 corresponds to thereference parameter-detecting means, and the second referenceparameter-detecting means.

The left and right front wheel speed sensors 80 and 81 detect the speedsof the left and right front wheels, to deliver signals indicative of therespective sensed left and light front wheel speeds to the ECU 2. TheECU 2 calculates the left and right front wheel speeds based on thesignals from the left and right front wheel speed sensors 80 and 81, andcalculates the arithmetic mean thereof as a non-drive wheel speedWs_ref. Further, the ECU 2 calculates the left and right rear wheelspeeds based on the signals from the left and right rear wheel speedsensors 82 and 83, and calculates the arithmetic mean thereof as a drivewheel speed Ws_act.

It should be noted that in the present embodiment, the left and rightfront wheel speed sensors 80 and 81 correspond to the first referenceparameter-detecting means, the non-drive wheel speed Ws_ref to the firstreference parameter, the left and right rear wheel speed sensors 82 and83 to the controlled variable-detecting means, and the drive wheel speedWs_act to the controlled variable and the wheel speed of the vehicle.

Further, as shown in FIG. 39, the control apparatus 1A includes atraction controller 200. As described hereinafter, the tractioncontroller 200 is provided for calculating the engine torque Trq astorque of the engine 3 which makes it possible to avoid the idling stateof the drive wheels, to thereby ensure the stability of the vehicle andenhance the acceleration performance of the engine 3 in a compatiblemanner. The traction controller 200 is implemented by the ECU 2. Itshould be noted that in the present embodiment, the traction controller200 corresponds to the control input-calculating means, and the enginetorque Trq corresponds to the control input and the output of the engine3.

As shown in FIG. 39, the traction controller 200 is comprised of atarget wheel speed-calculating section 201, a wheel speed feedbackcontroller 202, a maximum/minimum torque-calculating section 203, anormalization demand driving force-calculating section 204, amultiplication element 205, a feedforward torque-calculating section206, an addition element 207, and a torque correction value-calculatingsection 210.

First, the target wheel speed-calculating section 201 calculates atarget wheel speed Ws_cmd by the following equation (45). It should benoted that in the present embodiment, the target wheel speed-calculatingsection 201 corresponds to target value-setting means, and the targetwheel speed Ws_cmd corresponds to the target value.Ws _(—) cmd(k)=Ws _(—) ref(k)+OptSlip  (45)

In the above equation (45), OptSlip represents a predetermined slipoffset value which corresponds to a slip amount allowable between thedrive wheels and the non-drive wheels, and in the present embodiment, itis set to a fixed value (e.g. 10 km/h). It should be noted that the slipoffset value OptSlip may be determined by searching a map or apredetermined equation, according to a predetermined parameter (e.g. thenon-drive wheel speed Ws_ref, an estimated value of the frictionalresistance coefficient of a road surface, a detection signal from a yawrate sensor, a detection signal from a slip angle sensor mounted on thebody of the vehicle, etc.).

Further, the wheel speed feedback controller 202 calculates a torquefeedback value Trq_fb by a method, described hereinafter, based on thetarget wheel speed Ws_cmd and the drive wheel speed Ws_act. It should benoted that in the present embodiment, the torque feedback value Trq_fbcorresponds to the second input value.

Furthermore, the torque correction value-calculating section 210calculates a torque correction value Ktrq by a method, describedhereinafter, based on the torque feedback value Trq_fb, the engine speedNE, and the non-drive wheel speed Ws_ref. It should be noted that in thepresent embodiment, the torque correction value-calculating section 210corresponds to the model-modifying means and the corrected modificationvalue-calculating means, and the torque correction value Ktrqcorresponds to the corrected modification value.

On the other hand, the maximum/minimum torque-calculating section 203calculates a maximum torque Trq_max and a minimum torque Trq_min bysearching a map shown in FIG. 40 according to the engine speed NE. InFIG. 40, NEhigh represents a predetermined maximum allowable enginespeed (e.g. 7000 rpm). These values Trq_max and Trq_min represent themaximum value and the minimum value of the engine torque which can beachieved when the engine speed NE is equal to the associated enginespeed. Further, in this map, the minimum torque Trq_min is set to anegative value. This is because the minimum torque Trq_min correspondsto engine torque obtained in a state in which the accelerator pedal isnot stepped on, i.e. in an engine brake state during a deceleration fuelcut-off operation. It should be noted that in the present embodiment,the maximum torque Trq_max corresponds to the reference parameter, alimit value of the output of the engine 3, and the second referenceparameter.

Further, the normalization demand driving force-calculating section 204calculates a normalization demand driving force Ktrq_ap by searching amap shown in FIG. 41 according to the accelerator pedal opening AP. InFIG. 41, APmax represents the maximum value (100%) of the acceleratorpedal opening. Further, the normalization demand driving force Ktrq_aprepresents a value obtained by normalizing the normalization demanddriving force Ktrq_ap determined based on the accelerator pedal openingAP, with reference to a demand driving force Trq_apmax obtained whenAP=APmax holds, that is, a value which satisfies the equation,Ktrq_ap=Trq_ap÷Ktrq_apmax.

The multiplication element 205 calculates a corrected maximum torqueTrq_max_mod by the following equation (46). More specifically, thecorrected maximum torque Trq_max_mod is calculated by correcting themaximum torque Trq_max by the torque correction value Ktrq.Trq_max_mod(k)=Ktrq(k)·Trq_max(k)  (46)

Further, the feedforward torque-calculating section 206 calculates afeedforward torque Trq_ff by the following equation (47).Trq _(—) ff(k)=Ktrq _(—) ap(k){Trq_max_mod(k)−Ttrq_min(k)}+Ttrq_min(k)  (47)

It should be noted that in the present embodiment, the feedforwardtorque-calculating section 206 corresponds to the first inputvalue-calculating means, and the feedforward torque Trq_ff correspondsto the first input value. Further, calculating the feedforward torqueTrq_ff using the equations (46) and (47) corresponds to calculating thefirst input value using a modified correlation model.

Then, finally, the addition element 207 calculates the engine torque Trqby the following equation (48). More specifically, the engine torque Trqis calculated as the sum of the torque feedback value Trq_fb and thefeedforward torque Trq_ff.Trq(k)=Trq _(—) fb(k)+Trq _(—) ff(k)  (48)

Next, a description will be given of the aforementioned wheel speedfeedback controller 202. The wheel speed feedback controller 202calculates the torque feedback value Trq_fb with a control algorithmexpressed by the following equations (49) to (59), to which are applieda combination of a target value filter-type two-degree-of-freedomsliding mode control algorithm, and an adaptive disturbance observer.

$\begin{matrix}{{{Ws\_ cmd}{\_ f}(k)} = {{{{- {Rt}} \cdot {Ws\_ cmd}}{\_ f}( {k - 1} )} + {( {1 + {Rt}} ){Ws\_ cmd}(k)}}} & (49) \\{{{Et}(k)} = {{{Ws\_ act}(k)} - {{Ws\_ cmd}{\_ f}(k)}}} & (50) \\{{\sigma\;{t(k)}} = {{{Et}(k)} + {{St} \cdot {{Et}( {k - 1} )}}}} & (51) \\{{{Urch\_ t}(k)} = {{{- {Krch\_ t}} \cdot \sigma}\;{t(k)}}} & (52) \\{{{Unl\_ t}(k)} = {{- {Knl\_ t}} \cdot {{sgn}( {\sigma\;{t(k)}} )}}} & (53) \\{{\sigma\;{t\_ hat}(k)} = {{{Urch\_ t}( {k - 1} )} + {{Unl\_ t}( {k - 1} )} + {{Uls\_ t}( {k - 1} )}}} & (54) \\\begin{matrix}{{{Et\_ sig}(k)} = {{\sigma\;{t(k)}} - {\sigma\;{t\_ hat}(k)}}} \\{= {{\sigma\;{t(k)}} - {{Urch\_ t}( {k - 1} )} - {{Un\_ t}( {k - 1} )} - {{Uls\_ t}( {k - 1} )}}}\end{matrix} & (55) \\{{{Uls\_ t}(k)} = {{\lambda\;{t \cdot {Uls\_ t}}( {k - 1} )} + {\frac{Pt}{1 + {Pt}}\mspace{14mu}{Et\_ sig}(k)}}} & (56) \\{{{*{When}\mspace{14mu}{Uls\_ t}{\_ L}} < {{Uls\_ t}{\_ H}}}{{\lambda\; t} = 1}} & (57) \\{{{*{When}\mspace{14mu}{Uls\_ t}( {k - 1} )} \leqq {{Uls\_ t}{\_ L}\mspace{14mu}{or}\mspace{14mu}{Uls\_ t}{\_ H}} \leqq {{Uls\_ t}( {k - 1} )}}{{\lambda\; t} = {\lambda\;{tlmt}}}} & (58) \\{{{Trq\_ fb}(k)} = {{{Urch\_ t}(k)} + {{Unl\_ t}(k)} + {{Uls\_ t}(k)}}} & (59)\end{matrix}$

In the above control algorithm, first, a filtered value Ws_cmd_f of thetarget wheel speed is calculated with a first order lag type low passfilter algorithm expressed by the equation (49). In the equation (49),Rt represents a target value filter-setting parameter, and is set to avalue which satisfies the relationship of −1<Rt<0. In this case, thefollow-up speed of the filtered value Ws_cmd_f to the target wheel speedWs_cmd is determined by a value set to the target value filter-settingparameter Rt.

Then, a reaching law input Urch_t and a non-linear input Unl_t arecalculated with a control algorithm to which is applied a sliding modecontrol algorithm expressed by the following equations (50) to (53). Inthe equation (50), Et represents a follow-up error, and in the equation(51), at represents a switching function. Further, in the equation (51),St represents a switching function-setting parameter, and is set to avalue which satisfies the relationship pf−1<St<0. In this case, theconvergence rate of the follow-up error Et to 0 is designated by a valueset to the switching function-setting parameter St. Further, in theequation (52), Krch_t represents a predetermined reaching law gain, andin the equation (53), Knl_t represents a predetermined non-linear inputgain. Furthermore, in the equation (53), sgn(σt(k)) represents a signfunction, and the value thereof is set such that sgn(σt(k))=1 holds whenσt(k)≧0, and when σt(k)<0, sgn(σt(k))=−1 holds (it should be noted thatthe value thereof may be set such that sgn(σt(k))=0 holds when at(k)=0).

Then, a disturbance estimated value Uls_t is calculated with a controlalgorithm to which is applied an adaptive disturbance observer expressedby the equations (54) to (58). In the equation (54), σt_hat representsan estimated value of a switching function, and Uls_t represents adisturbance estimated value. The disturbance estimated value Uls_t iscalculated with a fixed gain identification algorithm expressed by theequations (55) and (56). In the equation (55), Et_sig represents anestimation error, and in the equation (56), Pt represents a fixedidentification gain.

Further, in the equation (56), λt represents a forgetting coefficient,and as shown in the equations (57) and (58), the value thereof is set to1 or a predetermined value λtlmt, according to the results ofcomparisons between the immediately preceding value Uls_t(k−1) of thedisturbance estimated value and predetermined upper and lower limitvalues Uls_t_H and Uls_t_L. The upper limit value Uls_t_H is set to apredetermined positive value, and the lower limit value Uls_t_L is setto a predetermined negative value, while the predetermined value λtlmtis set to a value which satisfies the relationship of 0<λtlmt<1.

Then, as shown in the equation (59), the torque feedback value Trq_fb isfinally calculated as the sum of the reaching law input Urch_t, thenon-linear input Unl_t, and the disturbance estimated value Uls_t.

As described above, the wheel speed feedback controller 202 calculatesthe torque feedback value Trq_fb with the control algorithm expressed bythe equations (49) to (59), and therefore the torque feedback valueTrq_fb is calculated as a value for causing the drive wheel speed Ws_actto converge to the filtered value Ws_cmd_f of the target wheel speed, inother words, as a value for causing the drive wheel speed Ws_act toconverge to the target wheel speed Ws_cmd. In this case, as describedhereinabove, the target wheel speed Ws_cmd is calculated by adding theslip offset value OptSlip to the non-drive wheel speed Ws_ref, so thatin a state of Ws_act≈Ws_cmd, Ws_ref−Ws_act≈OptSlip holds.

Further, the torque feedback value Trq_fb is calculated using theforgetting coefficient λt, and hence if the absolute value of theimmediately preceding value Uls_t(k−1) of the disturbance estimatedvalue is large, the above-described forgetting effect provided by theforgetting coefficient λ makes it possible to prevent the disturbanceestimated value Uls_t as an integral term, i.e. the torque feedbackvalue Trq_fb from being increased. As a result, as describedhereinafter, the torque correction value Ktrq calculated based on thetorque feedback value Trq_fb temporarily becomes improper, therebymaking it possible to prevent the feedforward torque Trq_ff fromtemporarily taking an improper value. In addition, the disturbanceestimated value Uls_t as the integral term in the algorithm forcalculating the torque feedback value Trq_fb can be prevented frominterfering with an integral value (disturbance estimated value Uls_v)in an algorithm, described hereinafter, for calculating the torquecorrection value Ktrq, to thereby prevent the integral terms fromexhibiting oscillating behaviors. This makes it possible to enhancecontrollability in a transient state. Further, if the absolute value ofthe immediately preceding value Uls_t(k−1) of the disturbance estimatedvalue is small, the forgetting coefficient λt is set to 1, and henceeven when the follow-up error Et has converged to 0, the torque feedbackvalue Trq_fb can be held at a value appropriate enough to compensate forthe follow-up error Et quickly, whereby it is possible to enhance theresponsiveness when the follow-up error Et starts to increase.

Next, the aforementioned torque correction value-calculating section 210will be described with reference to FIG. 42. The torque correctionvalue-calculating section 210 is provided for calculating the torquecorrection value Ktrq for use in correcting the maximum torque Trq_max.As shown in FIG. 42, the torque correction value-calculating section 210is comprised of an error weight-calculating section 211, a torqueerror-calculating section 212, a modified torque error-calculatingsection 213, a basic torque correction value-calculating section 214, atorque correction sensitivity-calculating section 215, and amultiplication element 216.

First, the error weight-calculating section 211 calculates an errorweight Wt by searching a map shown in FIG. 43 according to the enginespeed NE and the non-drive wheel speed Ws_ref. The error weight Wt takesa value obtained by normalizing a ratio ΔWs_act/ΔTrq between the amountΔWs_act of change in the drive wheel speed Ws_act and the amount ΔTrq ofchange in the engine torque, with reference to the absolute value|ΔWs_act_ref/ΔTrq_ref| of a ratio ΔWs_act_ref/ΔTrq_ref obtained at apredetermined drive wheel speed and a predetermined engine torque, thatis, a value which satisfies the equation,Wt=(ΔWs_act/ΔTrq)÷(|ΔWs_act_ref/ΔTrq_ref|).

The error weight Wt represents a probability of the torque error Etfbeing caused by a too large amount of the engine torque Trq, when it isassumed that the correlation between the engine speed NE and thefeedforward torque Trq_ff, that is, the correlation between the maximumtorque Trq_max and the feedforward torque Trq_ff is changed by a suddenincrease in the accelerator pedal opening AP, whereby the torque errorEtf, i.e. the slippage of a drive wheel is caused. More specifically,the error weight Wt is set to a larger value as the probability of thetorque error Etf being caused by a too large amount of the engine torqueTrq is higher. In other words, the error weight Wt is calculated as avalue which represents the degree of influence of the maximum torqueTrq_max on the torque error Etf. Further, since the degree of theinfluence of the maximum torque Trq_max on the torque error Etf alsovaries with the gear ratio of the transmission, in FIG. 51, the errorweight Wt is set according to the engine speed NE and the non-drivewheel speed Ws_ref.

In FIG. 43, Ws_ref 1 to Ws_ref 3 represent predetermined values of thenon-drive wheel speed, which satisfy the relationship of Ws_ref 1<Ws_ref2<Ws_ref 3. In this map, the error weight Wt is set to a smaller valueas the non-drive wheel speed Ws_ref is higher. This is because when thenon-drive wheel speed Ws_ref is high, the slippage of a drive wheel ismore difficult to occur as the gear ratio of the transmission is higher,and hence the error weight Wt is set to a smaller value to thereby makesmaller the amount of correction of the maximum torque Trq_max by thetorque correction value Ktrq in the decreasing direction. Further, theerror weight Wt is set such that it has the same tendency with respectto the engine speed NE as a torque curve in engine performance curveshas. This is because the error weight Wt is a value indicative of thedegree of influence of the maximum torque Trq_max on the torque errorEtf.

It should be noted that in the present embodiment, the errorweight-calculating section 211 corresponds to the influence degreeparameter-calculating means and the second influence degreeparameter-calculating means, and the error weight Wt corresponds to theinfluence degree parameter and the second influence degree parameter.Further, although the FIG. 43 map is provided for calculating the errorweight Wt according to the engine speed NE and the non-drive wheel speedWs_ref, the FIG. 43 map can be regarded to represent the correlationbetween the maximum torque Trq_max, the non-drive wheel speed Ws_ref,and the error weight Wt, since the maximum torque Trq_max is determinedbased on the engine speed NE, as described hereinbefore. Therefore, FIG.43 corresponds to the influence degree model and the second influencedegree model.

Further, the torque error-calculating section 212 calculates the torqueerror Etf by the following equation (60). It should be noted that in thepresent embodiment, the torque error-calculating section 212 correspondsto the error parameter-calculating means, and the torque error Etfcorresponds to the error parameter.Etf(k)=Trq _(—) fb(k)−Trq _(—) fb _(—) cmd(k)  (60)

In the above equation (60), Trq_fb_cmd represents a target torquefeedback value serving as a target of a torque feedback value Trq_fb,and is set to a fixed value (e.g. 0).

Further, the modified torque error-calculating section 213 calculates amodified torque error Wetrq by the following equation (61). It should benoted that in the present embodiment, the modified torqueerror-calculating section 213 correspond to the corrected errorparameter-calculating means, and the modified torque error Wetrqcorrespond to the corrected error parameter.Wetrq(k)=Wt(k)·Etf(k)  (61)

Next, the basic torque correction value-calculating section 214calculates a basic torque correction value Ktrq_bs with a controlalgorithm to which is applied a sliding mode control algorithm expressedby the following equations (62) to (69). It should be noted that in thepresent embodiment, the basic torque correction value-calculatingsection 214 corresponds to the modification value-calculating means, andthe basic torque correction value Ktrq_bs corresponds to themodification value.σv(k)=Wetrq(k)+Sv·Wetrq(k−1)  (62)Urch _(—) v(k)=−Krch _(—) v·σv(k)  (63)Unl _(—) v(k)=−Knl _(—) v·sgn(σv(k))  (64)Uadp _(—) v(k)=−Kadp _(—) v·δv(k)+Uadp _(—) v _(—) ini  (65)δv(k)=λv·δv(k−1)+σv(k)  (66)When Ktrq _(—) bs _(—) L<Ktrq _(—) bs(k−1)<Ktrq _(—) bs _(—) Hλv=1  (67)When Ktrq _(—) bs(k−1)≦Ktrq _(—) bs _(—) LorKtrq _(—) bs _(—) H≦Ktrq_(—) bs(k−1) λv=λvlmt  (68)Ktrq _(—) bs(k)=Urch _(—) v(k)+Unl _(—) v(k)+Uadp _(—) v(k)  (69)

In the above equation (62), σv represents a switching function, and Sv aswitching function-setting parameter which is set to a value satisfyingthe relationship of −1<Sv<St<0. The reason for thus setting the twoswitching function-setting parameters St and Sv will be describedhereinafter. In this case, the convergence rate of the modified torqueerrors Wetrq_(i) to 0 is designated by a value set to the switchingfunction-setting parameter Sv. Further, in the equation (63), Urch_vrepresents a reaching law input, and Krch_v represents a predeterminedreaching law gain. Furthermore, in the equation (64), Unl_v represents anon-linear input, and Knl_v represents a predetermined non-linear inputgain. Further, in the equation (64), sgn(σv(k)) represents a signfunction, and the value thereof is set such that sgn(σv(k))=1 holds whenσv(k)≧0, and when σv(k)<0, sgn(σv(k))=−1 holds (it should be noted thatthe value thereof may be set such that sgn(σv(k))=0 holds when σv(k)=0).

In the equation (65), Uadp_v represents an adaptive law input, andKadp_v represents a predetermined adaptive law gain. Further, in theequation (65), Uadp_v_ini represents the initial value of the adaptivelaw input, and is set to a fixed value (e.g. 1) such that the torquecorrection value Ktrq, which is the multiplication value, does notbecome a negative value. Furthermore, in the equation (65), δvrepresents the integral value of a switching function calculated by theequation (66). In the equation (66), λv represents a forgettingcoefficient, and as shown in the equations (67) and (68), the valuethereof is set to 1 or a predetermined value λvlmt, according to theresults of comparisons between the immediately preceding valueKtrq_bs(k−1) of the basic torque correction value and predeterminedupper and lower limit values Ktrq_bs_H and Ktrq_bs_L. The upper limitvalue Ktrq_bs_H is set to a predetermined positive value, and the lowerlimit value Ktrq_bs_L is set to a predetermined negative value, whilethe predetermined value λvlmt is set to a value which satisfies therelationship of 0<λvlmt<1, as described above.

Further, as shown in the equation (69), the basic torque correctionvalue Ktrq_bs is calculated as the sum of the reaching law input Urch_v,the non-linear input Unl_v, and the adaptive law input Uadp_v.

As described above, the basic torque correction value-calculatingsection 214 calculates the basic torque correction value Ktrq_bs withthe control algorithm expressed by the equations (62) to (69), andtherefore the basic torque correction value Ktrq_bs is calculated as avalue for causing the modified torque error Wetrq to converge to 0, inother words, as a value for causing the torque feedback value Trq_fb toconverge to the target torque feedback value Trq_fb_cmd.

Further, the basic torque correction value Ktrq_bs is calculated usingthe forgetting coefficient λv, and hence when the absolute value of theimmediately preceding value Ktrq_bs(k−1) of the basic torque correctionvalue is large, the above-described forgetting effect provided by theforgetting coefficient λ makes it possible to prevent the disturbanceestimated value Uls_v as the integral term, i.e. the basic torquecorrection value Ktrq_bs from being increased, thereby making itpossible to prevent the feedforward torque Trq_ff from temporarilytaking an improper value. In addition, the basic torque correction valueKtrq_bs, that is, the disturbance estimated value Uls_v as the integralterm in the algorithm for calculating the torque correction value Ktrqcan be prevented from interfering with the integral value Uls_t in thealgorithm for calculating the above-described torque feedback valueTrq_fb, to thereby prevent the integral terms from exhibitingoscillating behaviors. This makes it possible to enhance controllabilityin a transient state. Further, if the absolute value of the immediatelypreceding value Ktrq_bs(k−1) of the basic torque correction value issmall, the forgetting coefficient λv is set to 1, and hence even whenthe torque error Etf has converged to 0, the torque feedback valueTrq_fb can be held at a proper value which is capable of compensatingfor the follow-up error Et quickly. This makes it possible to enhancethe responsiveness when the modified torque errors Wetrq start toincrease.

On the other hand, the torque correction sensitivity-calculating section215 calculates a torque correction sensitivity Rtrq by searching a mapshown in FIG. 44 according to the engine speed NE and the non-drivewheel speed Ws_ref. Similarly to the above-described error weight Wt,the torque correction sensitivity Rtrq takes a value obtained bynormalizing a ratio ΔWs_act/ΔTrq between the amount ΔWs_act of change inthe drive wheel speed Ws_act and the amount ΔTrq of change in the enginetorque, with reference to the absolute value |ΔWs_act_ref/ΔTrq_ref| ofthe ratio ΔWs_act_ref/ΔTrq_ref obtained at the predetermined drive wheelspeed and the predetermined engine torque.

In FIG. 44, curves indicated by solid lines represent the values of thetorque correction sensitivity Rtrq, and curves indicated by broken linesrepresent the values of the above-described error weight Wt, forcomparison. As is clear from the comparison between the two curves, inthis map, the torque correction sensitivity Rtrq is set to haveapproximately the same tendency as that of the error weight Wt. Thereason for this is the same as given in the description of the FIG. 43map.

As described above, since the torque correction sensitivity Rtrq iscalculated by the same method as employed for the calculation of theerror weight Wt, the torque correction sensitivity Rtrq is calculated asa value indicative of the degree of the influence of the maximum torqueTrq_max on the torque error Etf. Further, as described hereinabove, thedegree of the influence of the maximum torque Trq_max on the torqueerror Etf also varies with the gear ratio of the transmission, and hencein FIG. 44, the torque correction sensitivity Rtrq is set according tothe engine speed NE and the non-drive wheel speed Ws_ref.

Further, in FIG. 44, the torque correction sensitivity Rtrq is set to avalue equal to the value of the error weight Wt in a low non-drive wheelspeed region and at the same time in a low-to-medium engine speedregion, that is, in a region where the traction control is easy tooperate, and in the other regions, the torque correction sensitivityRtrq is set to a smaller value than the value of the error weight Wt.This is because when the amount of correction of the maximum torqueTrq_max by the torque correction value Ktrq in the decreasing directionis too small, there can occur slippage of the drive wheels. To avoidthis problem, the values of the torque correction sensitivity Rtrq areset as above.

It should be noted that in the present embodiment, the torque correctionsensitivity-calculating section 215 corresponds to the first influencedegree parameter-calculating means, and the torque correctionsensitivity Rtrq corresponds to the first influence degree parameter.Further, although the FIG. 44 map is provided for calculating the torquecorrection sensitivity Rtrq according to the engine speed NE and thenon-drive wheel speed Ws_ref, it can be regarded to represent thecorrelation between the maximum torque Trq_max, the non-drive wheelspeed Ws_ref, and the torque correction sensitivity Rtrq since themaximum torque Trq_max is determined according to the engine speed NE,as described hereinbefore. Therefore, the FIG. 44 map corresponds to thefirst influence degree model.

On the other hand, the multiplication element 216 calculates the torquecorrection value Ktrq by the following equation (70). More specifically,the torque correction value Ktrq is calculated by correcting the basictorque correction value Ktrq_bs by the torque correction sensitivityRtrq.Ktrq(k)=Rtrq(k)·Ktrq _(—) bs(k)  (70)

As described hereinabove, the control apparatus 1A according to thepresent embodiment calculates the engine torque Trq by the tractioncontroller 200, and although not shown, carries out the variablemechanism control process, the air-fuel ratio control process, and theignition timing control process so as to obtain the engine torque Trq.

Next, a description will be given of control results obtained when thetraction control is performed by the control apparatus 1A according tothe second embodiment configured as described above. FIG. 45 shows anexample of control results obtained by the control apparatus 1A when theacceleration/deceleration of the vehicle is repeatedly performed on aroad surface having a small frictional resistance. FIG. 46 shows, forcomparison with the FIG. 45 example, an example (hereinafter referred toas “the comparative example”) of control results obtained when thetorque correction value Ktrq is held at 1, i.e. when the maximum torqueTrq_max is directly used as the corrected maximum torque Trq_max_mod.

Referring to FIGS. 45 and 46, when a comparison is made between theexample and the comparative example as to changes in the feedforwardtorque Trq_ff and the torque feedback value Trq_fb within a time periodfrom the start of acceleration through the start of deceleration (fromt30 to t31, from t32 to t33, and from t34 to t35, and form t40 to t41,from t42 to t43, and from t44 to t45), it is understood that the twovalues Trq_ff and Trq_fb are both made smaller in the example accordingto the present embodiment than in the comparative example, whereby thepresent embodiment is enhanced in controllability.

Further, when another comparison is made between the example and thecomparative example as to changes in the drive wheel speed Ws_act withrespect to those in the target wheel speed Ws_cmd after the start of thedeceleration, it is understood that the degree of deviation of the drivewheel speed Ws_act from the target wheel speed Ws_cmd, i.e. the controlerror is suppressed to a smaller value in the example of the controlresults according to the present embodiment than in the comparativeexample, whereby the present embodiment is enhanced in the controlaccuracy.

As described hereinabove, according to the control apparatus 1A of thesecond embodiment, the error weight Wt is calculated using the map shownin FIG. 43, i.e. the response surface model, and the modified torqueerror Wetrq is calculated by correcting (modifying) the torque error Etfby the error weight Wt. As described hereinabove, the error weight Wt iscalculated as a value indicative of the probability of the torque errorEtf being caused by the too large amount of the engine torque Trq, inother words, the degree of influence of the maximum torque Trq_max onthe torque error Etf, and therefore the modified torque error Wetrq iscalculated as a value on which is reflected the degree of influence ofthe maximum torque Trq_max on the torque error Etf.

Further, the basic torque correction value Ktrq_bs is calculated suchthat the modified torque error Wetrq calculated as above is caused toconverge to 0, and the torque correction value Ktrq is calculated bymultiplying the basic torque correction value Ktrq_bs by the torquecorrection sensitivity Rtrq. The feedforward torque Trq_ff is calculatedby the equation (47), using the corrected maximum torque Trq_max_modobtained by correcting the maximum torque Trq_max by the torquecorrection value Ktrq. Therefore, the torque error Etf can be properlyand quickly compensated for just enough by the feedforward torque Trq_ffcalculated using the corrected maximum torque Trq_max_mod and theequation (47), even under a condition where the correlation between themaximum torque Trq_max and the feedforward torque Trq_ff is changed byunpredictable changes of conditions other than disturbance, such as ageddegradation of output characteristics of the engine 3, variationsbetween individual engines, changes in the degrees of wear of tires, andchanges in the frictional resistance of road surfaces, causing thetorque error Etf, i.e. the slippage of the drive wheels to be liable totemporarily increase. This makes it possible to ensure higher-levelcontrol accuracy of the wheel speed than a gain schedule correction (ormodification) method. In short, a high-level traction control can berealized.

If the feedforward torque Trq_ff is calculated assuming that the errorweight=1 and Etf=Wetrq hold, when the torque error Etf is caused mainlyby the change in the above-described correlation between the maximumtorque Trq_max and the feedforward torque Trq_ff, i.e. when the degreeof influence of the maximum torque Trq_max on the torque error Etf islarge, the torque error Etf, i.e. the slippage of the drive wheels canbe properly compensated for by the thus calculated the feedforwardtorque Trq_ff. However, when the degree of influence of the maximumtorque Trq_max on the torque error Etf is small, i.e. when the torqueerror Etf, i.e. the slippage of the drive wheels is caused mainly by adisturbance or the like other than the above-described correlation, itis impossible to properly compensate for the torque error Etf, i.e. theslippage of the drive wheels, which results in overcompensation orundercompensation for the same. Therefore, by using the above-describederror weight Wt, the torque error Etf, i.e. the slippage of the drivewheels can be properly compensated for just enough by the feedforwardtorque Trq_ff.

In addition, since the feedforward torque Trq_ff is calculated using theequation (47) indicative of the correlation between the correctedmaximum torque Trq_max_mod and the feedforward torque Trq_ff, theslippage of the drive wheels can be compensated for more quickly thanwhen the slippage of the drive wheels is compensated for by the torquefeedback value Trq_fb calculated with the feedback control algorithm. Asdescribed above, even under a condition where the torque error Etf, i.e.the slippage of the drive wheels is temporarily increased by a change inthe correlation between the corrected maximum torque Trq_max_mod and thefeedforward torque Trq_ff, it is possible to compensate for the slippageof the drive wheels properly and quickly, thereby making it possible toensure high-level accuracy of control.

Furthermore, although the degree of the influence of the maximum torqueTrq_max on the torque error Etf also varies with the gear ratio of thetransmission, as described above, the error weight Wt is calculatedaccording to the engine speed NE and the non-drive wheel speed Ws_ref.Therefore, the feedforward torque Trq_ff can be calculated so as tocompensate for the slippage of the drive wheels while reflectinginfluence of the engine speed NE and the non-drive wheel speed Ws_ref onthe torque error Etf. This makes it possible to further enhance theaccuracy of control.

Further, the torque correction sensitivity Rtrq is calculated as a valuewhich represents the degree of the influence of the maximum torqueTrq_max on the torque error Etf, and the torque correction value Ktrq iscalculated by multiplying the basic torque correction value Ktrq_bs bythe torque correction sensitivity Rtrq, so that as described above, itis possible to prevent the basic torque correction value Ktrq_bs fromovercompensating for the torque error Etf under the condition where thedegree of the influence of the maximum torque Trq_max on the torqueerror Etf is small. In addition, the torque correction sensitivity Rtrqis set to a value equal to the value of the error weight Wt in theregion where the traction control is easy to operate, and in the otherregions, the torque correction sensitivity Rtrq is set to a smallervalue than the value of the error weight Wt, whereby it is possible toprevent the slippage of the drive wheels from being caused due to theamount of correction of the maximum torque Trq_max by the torquecorrection value Ktrq in the decreasing direction being too small.

Further, in the algorithm [equations (49) to (59)] for calculating thetorque feedback value Trq_fb, and the algorithm [equations (62) to (69)]for calculating the basic lift correction value Ktrq_bs, the switchingfunction-setting parameters St and Sv are set to values which satisfythe relationship of −1<Sv<St<0. Therefore, the convergence rate of themodified torque error Wetrq to 0 is lower than the convergence rate ofthe follow-up error Et to 0, which prevents the two response-specifyingcontrol algorithms from interfering with each other. Particularly, thetorque correction value Ktrq is calculated based on the torque feedbackvalue Trq_fb, and hence it is necessary to modify the correlation modelby the torque correction value Ktrq at a rate lower than the convergencerate of the follow-up error Et to 0. However, by setting the switchingfunction-setting parameters St and Sv as described above, it is possibleto realize the modification at the required rate. This makes it possibleto prevent the control system from exhibiting an oscillating behaviordue to the interference between the response-specifying controlalgorithms, thereby making it possible to ensure the stability of thecontrol system.

It should be noted that although in the second embodiment, thefeedforward torque-calculating section 206 calculates the feedforwardtorque Trq_ff by the aforementioned equation (47), by way of example,the feedforward torque-calculating section 206 may be configured tocalculate the feedforward torque Trq_ff by the following equations (71)to (73) in place of the equation (47).Trq _(—) ff_temp(k)=Ktrq _(—)ap(k){Trq_max(k)−Ttrq_min(k)}+Ttrq_min(k)  (71)When Trq _(—) ff_temp(k)≦Trq_max_mod(k)Trq _(—) ff(k)=Trq _(—)ff_temp(k)  (72)When Trq _(—) ff_temp(k)>Trq_max_mod(k) Trq _(—)ff(k)=Trq_max_mod(k)  (73)

In the above equation (71), Trq_ff_temp represents the provisional valueof the feedforward torque. As shown in the equations (72) and (73), alimiting process is performed on the provisional value Trq_ff_temp usingthe corrected maximum torque Trq_max_mod as an upper limit value,whereby the feedforward torque Trq_ff is calculated. Also when the aboveequations (71) to (73) are used as the algorithm for calculating thefeedforward torque Trq_ff, it is possible to obtain the sameadvantageous effects as provided by the use of the aforementionedequation (47).

Further, although in the second embodiment, the control algorithmexpressed by the aforementioned equations (62) to (69) is used as thealgorithm for calculating the basic torque correction value Ktrq_bs, thebasic torque correction value Ktrq_bs may be calculated, in place of theabove control algorithm, with a control algorithm expressed by thefollowing equations (74) to (83), to which are applied a combination ofan adaptive disturbance observer and a sliding mode control algorithm.

$\begin{matrix}{{\sigma\;{v(k)}} = {{{Wetrq}(k)} + {{Sv} \cdot {{Wetrq}( {k - 1} )}}}} & (74) \\{{{Urch\_ v}(k)} = {{{- {Krch\_ v}} \cdot \sigma}\;{v(k)}}} & (75) \\{{{Unl\_ v}(k)} = {{- {Knl\_ v}} \cdot {{sgn}( {\sigma\;{v(k)}} )}}} & (76) \\{{\sigma\;{v\_ hat}(k)} = {{{Urch\_ v}( {k - 1} )} + {{Unl\_ v}( {k - 1} )} + {{Uls\_ v}( {k - 1} )}}} & (77) \\\begin{matrix}{{{Ev\_ sig}(k)} = {{\sigma\;{v(k)}} - {\sigma\;{v\_ hat}(k)}}} \\{= {{\sigma\;{v(k)}} - {{Urch\_ v}( {k - 1} )} - {{Unl\_ v}( {k - 1} )} - {{Uls\_ v}( {k - 1} )}}}\end{matrix} & (78) \\{{{Uls\_ v}(k)} = {{{dUls\_ v}( {k - 1} )} + {{Uls\_ v}{\_ ini}}}} & (79) \\{{{dUls\_ v}(k)} = {{\lambda\;{v \cdot {dUls\_ v}}( {k - 1} )} + {\frac{Pv}{1 + {Pv}}{Ev\_ sig}(k)}}} & (80) \\{{{*{When}\mspace{14mu}{Ktrq\_ bs}{\_ L}} < {{Ktrq\_ bs}( {k - 1} )} < {{Ktrq\_ bs}{\_ H}}}{{\lambda\; v} = 1}} & (81) \\{{{*{When}\mspace{14mu}{Ktrq\_ bs}( {k - 1} )} \leqq {{Ktrq\_ bs}{\_ L}\mspace{14mu}{or}\mspace{14mu}{Ktrq\_ bs}{\_ H}} \leqq {{Ktrq\_ bs}( {k - 1} )}}{{\lambda\; v} = {\lambda\;{vlmt}}}} & (82) \\{{{Ktrq\_ bs}(k)} = {{{Urch\_ v}(k)} + {{Unl\_ v}(k)} + {{Uls\_ v}(k)}}} & (83)\end{matrix}$

In the above equation (77), σv_hat represents an estimated value of aswitching function, and Uls_v represents a disturbance estimated value.The disturbance estimated value Uls_v is calculated with a fixed gainidentification algorithm to which is applied a δ correcting methodexpressed by the equations (77) to (82). In the equation (78), Ev_sigrepresents an estimation error, and in the equation (79), Uls_v_inirepresents the initial value of the disturbance estimated value Uls_v.Further, in the equation (79), dUls_v represents a modification term,and is calculated by the equations (80) to (82). In the equation (80),Pv represents a fixed identification gain.

Further, as shown in the equation (83), the basic torque correctionvalue Ktrq_bs is calculated as the sum of the reaching law input Urch_v,the non-linear input Unl_v, and the disturbance estimated value Uls_v.It should be noted that the equations (77) to (82) express an algorithmwith which the disturbance estimated value Uls_v of the adaptivedisturbance observer is calculated.

According to the control algorithm configured as above, it is possibleto obtain the same advantageous effects as provided by the controlalgorithm expressed by the aforementioned equations (62) to (69). Morespecifically, in the equation (80), the immediately preceding valuedUls_v (k−1) of the modification term is multiplied by the forgettingcoefficient λv, and if the absolute value of the basic torque correctionvalue Ktrq_bs is large, the forgetting coefficient λv is set to a valuewithin the range of 0<λ<1. Therefore, the aforementioned forgettingeffect provided by the forgetting coefficient λ makes it possible toprevent the disturbance estimated value Uls_v as the integral term, i.e.the basic torque correction value Ktrq_bs from being increased, therebymaking it possible to prevent the feedforward torque Trq_ff fromtemporarily taking an improper value. In addition, the basic torquecorrection value Ktrq_bs, that is, the disturbance estimated value Uls_vas the integral term in the algorithm for calculating the torquecorrection value Ktrq can be prevented from interfering with theintegral value Uls_t in the algorithm for calculating theabove-described torque feedback value Trq_fb, to thereby prevent theintegral terms from exhibiting oscillating behaviors. This makes itpossible to enhance controllability in a transient state. Further, ifthe absolute value of the immediately preceding value Ktrq_bs(k−1) ofthe basic torque correction value is small, the forgetting coefficientλv is set to 1, and hence even when the modified torque error Wetrqbecomes close to 0, the basic torque correction value Ktrq_bs can beheld at a proper value. This makes it possible to enhance theresponsiveness when the modified torque error Wetrq start to increase,thereby making it possible to enhance the control accuracy.

In addition, the disturbance estimated value Uls_v is calculated withthe fixed gain identification algorithm of the adaptive disturbanceobserver, to which is applied the 6 correcting method, and hencecompared with the control algorithm according to the second embodimentwhich employs the adaptive law input Uadp_v, it is possible to furtherenhance the capability of suppressing the integral fluctuation behaviorand the overshooting behavior of the basic torque correction valueKtrq_bs.

On the other hand, although in the second embodiment, the maximum torqueTrq_maxis is regarded as the reference parameter and the secondreference parameter, by way of example, the engine speed NE may beregarded as the reference parameter and the second reference parameterin the second embodiment. In this case, calculating the feedforwardtorque Trq_ff using the FIG. 40 map and the equations (46) and (47)corresponds to calculating the first input using a modified correlationmodel, and the FIG. 43 map corresponds to the influence degree model andthe second and influence degree model, while the FIG. 44 map correspondsto the first influence degree model.

Further, although in the second embodiment, the feedforward torqueTrq_ff is calculated using the equations (46) and (47) as thecorrelation model, by way of example, the correlation model for use incalculation of the feedforward torque Trq_ff is not limited to this, butany other suitable calculating equations and maps may be used. Forexample, the feedforward torque Trq_ff may be calculated using anequation in which the corrected maximum torque Trq_max_mod and thenormalization demand driving force Ktrq_ap in the equation (47) arereplaced by the maximum torque Trq_max and a value Ktrq·Ktrq_ap,respectively. Further, the feedforward torque Trq_ff may be calculatedusing a calculating equation in which the corrected maximum torqueTrq_max_mod and the normalization demand driving force Ktrq_ap in theequation (47) are replaced by the maximum torque Trq_max and a valuewhich is obtained by performing a limiting process using the torquecorrection value Ktrq as an upper limit value on the normalizationdemand driving force Ktrq_ap, respectively.

Furthermore, although in the second embodiment, the equations (49) to(59) are used as the control algorithm to which is applied a firstresponse-specifying control algorithm, by way of example, the firstresponse-specifying control algorithm of the present invention is notlimited to this, but any other suitable response-specifying controlalgorithm may be used insofar as it specifies the convergence rate ofthe difference between the controlled variable and the target value to0. For example, a control algorithm to which is applied a back-steppingcontrol algorithm may be used. In this case as well, it is possible toobtain the same advantageous effects as provided by the controlalgorithm expressed by the equations (49) to (59) in the secondembodiment.

On the other hand, although in the second embodiment, the equations (62)to (69) are used as the control algorithm to which is applied a secondresponse-specifying control algorithm, by way of example, the secondresponse-specifying control algorithm of the present invention is notlimited to this, but any other suitable response-specifying controlalgorithm may be used insofar as it specifies the convergence rate ofthe corrected error parameter to 0. For example, a control algorithm towhich is applied a back-stepping control algorithm may be used. In thiscase as well, it is possible to obtain the same advantageous effects asprovided by the control algorithm expressed by the equations (62) to(69) in the second embodiment.

Further, although in the second embodiment, the error weight Wt iscalculated by searching the FIG. 43 map according to the engine speed NEand the non-drive wheel speed Ws_ref, by way of example, the method ofcalculating the error weight Wt is not limited to this. For example, inplace of the map shown in FIG. 43, there may be used a map in which thevalue of the error weight Wt is set in advance with respect to theaverage value of the drive wheel speed Ws_act and the non-drive wheelspeed Ws_ref, and the engine speed NE. Further, a map may be used inwhich each of values of the error weight Wt is set in advance withrespect to a larger (or smaller) one of the drive wheel speed Ws_act andthe non-drive wheel speed Ws_ref, and the engine speed NE. Furthermore,a map may be used in which each value of the error weight Wt is set inadvance with respect to the target wheel speed Ws_cmd and the enginespeed NE.

Furthermore, although in the second embodiment, the maps shown in FIGS.43 and 44 are used when the error weight Wt and the torque correctionsensitivity Rtrq are calculated during the traction control of theengine 3 with the automatic transmission, by way of example, this is notlimitative, but when traction control is carried out for an engine witha manual transmission, or for an engine with a so-called automatic MT inwhich an actuator instead of a manual operating force performs the speedvarying operation, in place of the maps shown in FIGS. 43 and 44, theremay be used a plurality of two-dimensional maps (i.e. tables) in whichthe values of the error weight Wt and the torque correction sensitivityRtrq are set in advance with respect to the engine speed NE on a gearratio-by-gear ratio basis, respectively.

Further, although in the second embodiment, the torque correctionsensitivity Rtrq is calculated using the FIG. 44 map as a correlationmodel, by way of example, this is not limitative, but the torquecorrection sensitivity Rtrq may be calculated using the FIG. 43 map inplace of the FIG. 44 map. That is, the torque correction sensitivityRtrq may be calculated as a value equal to the weight error Wt. Inaddition, in the equation (70), the torque correction sensitivity Rtrqmay be set to 1 such that Ktrq=Ktrq_bs holds. That is, the basic torquecorrection value Ktrq_bs may be used as the torque correction valueKtrq.

Further, although in the second embodiment, the correlation model ismodified by the method of correcting (modifying) the maximum torqueTrq_max as the second reference parameter by the torque correction valueas the corrected modification value, by way of example, the method ofmodifying the correlation model, according to the present invention, isnot limited to this, but any other suitable method may be used insofaras it is capable of modifying the correlation model. For example, theremay be used a method of modifying the first input value by the correctedmodification value.

Furthermore, although in the second embodiment, the wheel speed (morespecifically the drive wheel speed Ws_act) is used as the controlledvariable, by way of example, this is not limitative, but the controlapparatus according to the present invention may be configured such thatthe engine speed NE is used as the controlled variable to control thecontrolled variable to a target value while taking the gear ratio of thetransmission and the sliding amount of the clutch into account. In thiscase as well, it is possible to obtain the same advantageous effects asprovided by the control apparatus 1A according to the second embodiment.

Further, although in the first embodiment, the control apparatusaccording to the present invention is applied to a control apparatuswhich carries out air-fuel ratio control, and in the second embodiment,the control apparatus according to the present invention is applied to acontrol apparatus which carries out traction control, by way of example,this is not limitative, but it may be applied to any suitable controlapparatuses for various industrial apparatuses, which calculate a firstinput value for feedforward control of a controlled variable, accordingto reference parameters, by using a correlation model representative ofthe correlation between the reference parameters and the first inputvalue, calculates a second input value for use in performing feedbackcontrol of the controlled variable such that the controlled variable iscaused to converge to a target controlled variable, with a predeterminedfeedback control algorithm, and calculates a control input based on thefirst input value and the second input value.

Furthermore, although in the first and second embodiments, themodification values (correction values) for modifying the referenceparameters are calculated so as to modify the correlation model, by wayof example, modification values for modifying the first input value maybe calculated with the control algorithms according to the first andsecond embodiments.

It is further understood by those skilled in the art that the foregoingare preferred embodiments of the invention, and that various changes andmodifications may be made without departing from the spirit and scopethereof.

1. A control apparatus for controlling a controlled variable of acontrolled object by a control input, comprising: controlledvariable-detecting means for detecting the controlled variable;reference parameter-detecting means for detecting a reference parameterof the controlled object other than the controlled variable of thecontrolled object; target value-setting means for setting a target valueserving as a target to which the controlled variable is controlled; andcontrol input-calculating means for calculating a first input value forfeedforward control of the controlled variable, according to thereference parameter, using a correlation model representative of acorrelation between the reference parameter and the first input value,calculating a second input value for performing feedback control of thecontrolled variable such that the controlled variable is caused toconverge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein said control input-calculatingmeans comprises: error parameter-calculating means for calculating anerror parameter indicative of a control error to be compensated for bythe first input value, based on the controlled variable and the targetvalue; influence degree parameter-calculating means for calculating aninfluence degree parameter indicative of a degree of influence of thereference parameter on the error parameter by using an influence degreemodel representative of a correlation between the influence degreeparameter and the reference parameter; corrected errorparameter-calculating means for calculating a corrected error parameterby correcting the error parameter by the influence degree parameter;model-modifying means for modifying the correlation model according tothe corrected error parameter; and first input value-calculating meansfor calculating the first input value using the modified correlationmodel.
 2. A control apparatus as claimed in claim 1, wherein thepredetermined feedback control algorithm is an algorithm to which isapplied a predetermined first response-specifying control algorithm thatspecifies a convergence rate of a difference between the controlledvariable and the target value to 0, wherein said model-modifying meanscalculates a modification value with an algorithm to which is applied apredetermined second response-specifying control algorithm thatspecifies a convergence rate of the corrected error parameter to 0, andmodifies the correlation model by the modification value, and wherein inthe predetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.
 3. A controlapparatus as claimed in claim 1, wherein the controlled object is aninternal combustion engine in which an amount of intake air drawn into acylinder of the engine is changed by a variable intake mechanism, asdesired, the controlled variable being an air-fuel ratio of a mixture inthe engine, the control input being an amount of fuel to be supplied tothe engine, the reference parameter being an operating conditionparameter indicative of an operating condition of the variable intakemechanism.
 4. A control apparatus as claimed in claim 1, wherein thecontrolled object is a vehicle using the engine as a drive sourcethereof, the controlled variable being a wheel speed of the vehicle, thecontrol input being an output of the engine, the reference parameterbeing one of a limit value of the output of the engine and a rotationalspeed of the engine.
 5. A control apparatus for controlling a controlledvariable of a controlled object by a control input, comprising:controlled variable-detecting means for detecting the controlledvariable; first reference parameter-detecting means for detecting afirst reference parameter of the controlled object other than thecontrolled variable of the controlled object; second referenceparameter-detecting means for detecting a second reference parameter ofthe controlled object other than the controlled variable and the firstreference parameter of the controlled object; target value-setting meansfor setting a target value serving as a target to which the controlledvariable is controlled; and control input-calculating means forcalculating a first input value for feedforward control of thecontrolled variable, according to the first reference parameter and thesecond reference parameter, using a correlation model representative ofa correlation between the first reference parameter, the secondreference parameter, and the first input value, calculating a secondinput value for performing feedback control of the controlled variablesuch that the controlled variable is caused to converge to the targetvalue, with a predetermined feedback control algorithm, and calculatingthe control input based on the first input value and the second inputvalue, wherein said control input-calculating means comprises: errorparameter-calculating means for calculating an error parameterindicative of a control error to be compensated for by the first inputvalue, based on the controlled variable and the target value;modification value-calculating means for calculating a modificationvalue for modifying the correlation model according to the errorparameter; first influence degree parameter-calculating means forcalculating a first influence degree parameter indicative of a degree ofinfluence of the first reference parameter on the error parameter, usinga first influence degree model representative of a correlation betweenthe first influence degree parameter and the first reference parameter;corrected modification value-calculating means for calculating acorrected modification value by correcting the modification value by thefirst influence degree parameter; model-modifying means for modifyingthe correlation model according to the corrected modification value; andfirst input value-calculating means for calculating the first inputvalue using the modified correlation model.
 6. A control apparatus asclaimed in claim 5, further comprising: second influence degreeparameter-calculating means for calculating a second influence degreeparameter indicative of a degree of influence of the second referenceparameter on the error parameter, using a second influence degree modelrepresentative of a correlation between the second influence degreeparameter and the second reference parameter; and corrected errorparameter-calculating means for calculating a corrected error parameterby correcting the error parameter by the second influence degreeparameter; wherein said modification value-calculating means calculatesthe modification value according to the corrected error parameter.
 7. Acontrol apparatus as claimed in claim 6, wherein the predeterminedfeedback control algorithm is an algorithm to which is applied apredetermined first response-specifying control algorithm for specifyinga convergence rate of a difference between the controlled variable andthe target value to 0, wherein said modification value-calculating meanscalculates the modification value with an algorithm to which is applieda predetermined second response-specifying control algorithm thatspecifies a convergence rate of the corrected error parameter to 0, andwherein in the predetermined second response-specifying controlalgorithm, the convergence rate of the corrected error parameter to 0 isset such that it becomes lower than the convergence rate of thedifference to 0 in the predetermined first response-specifying controlalgorithm.
 8. A control apparatus as claimed in claim 5, wherein thecontrolled object is an internal combustion engine in which an amount ofintake air drawn into a cylinder of the engine is changed by a variableintake mechanism, as desired, the controlled variable being an air-fuelratio of a mixture in the engine, the control input being an amount offuel to be supplied to the engine, the second reference parameter beingan operating condition parameter indicative of an operating condition ofthe variable intake mechanism.
 9. A control apparatus as claimed inclaim 5, wherein the controlled object is a vehicle using the engine asa drive source thereof, the controlled variable being a wheel speed ofthe vehicle, the control input being an output of the engine, the secondreference parameter being one of a limit value of the output of theengine and a rotational speed of the engine.
 10. A method of controllinga controlled variable of a controlled object by a control input,comprising: a controlled variable-detecting step of detecting thecontrolled variable; a reference parameter-detecting step of detecting areference parameter of the controlled object other than the controlledvariable of the controlled object; a target value-setting step ofsetting a target value serving as a target to which the controlledvariable is controlled; and a control input-calculating step ofcalculating a first input value for feedforward control of thecontrolled variable, according to the reference parameter, using acorrelation model representative of a correlation between the referenceparameter and the first input value, calculating a second input valuefor performing feedback control of the controlled variable such that thecontrolled variable is caused to converge to the target value, with apredetermined feedback control algorithm, and calculating the controlinput based on the first input value and the second input value, whereinsaid control input-calculating step comprises: an errorparameter-calculating step of calculating an error parameter indicativeof a control error to be compensated for by the first input value, basedon the controlled variable and the target value; an influence degreeparameter-calculating step of calculating an influence degree parameterindicative of a degree of influence of the reference parameter on theerror parameter by using an influence degree model representative of acorrelation between the influence degree parameter and the referenceparameter; a corrected error parameter-calculating step of calculating acorrected error parameter by correcting the error parameter by theinfluence degree parameter; a model-modifying step of modifying thecorrelation model according to the corrected error parameter; and afirst input value-calculating step of calculating the first input valueusing the modified correlation model.
 11. A method claimed in claim 10,wherein the predetermined feedback control algorithm is an algorithm towhich is applied a predetermined first response-specifying controlalgorithm that specifies a convergence rate of a difference between thecontrolled variable and the target value to 0, wherein saidmodel-modifying step includes calculating a modification value with analgorithm to which is applied a predetermined second response-specifyingcontrol algorithm that specifies a convergence rate of the correctederror parameter to 0, and modifying the correlation model by themodification value, and wherein in the predetermined secondresponse-specifying control algorithm, the convergence rate of thecorrected error parameter to 0 is set such that it becomes lower thanthe convergence rate of the difference to 0 in the predetermined firstresponse-specifying control algorithm.
 12. A method as claimed in claim10, wherein the controlled object is an internal combustion engine inwhich an amount of intake air drawn into a cylinder of the engine ischanged by a variable intake mechanism, as desired, the controlledvariable being an air-fuel ratio of a mixture in the engine, the controlinput being an amount of fuel to be supplied to the engine, thereference parameter being an operating condition parameter indicative ofan operating condition of the variable intake mechanism.
 13. A method asclaimed in claim 10, wherein the controlled object is a vehicle usingthe engine as a drive source thereof, the controlled variable being awheel speed of the vehicle, the control input being an output of theengine, the reference parameter being one of a limit value of the outputof the engine and a rotational speed of the engine.
 14. A method ofcontrolling a controlled variable of a controlled object by a controlinput, comprising: a controlled variable-detecting step of detecting thecontrolled variable; a first reference parameter-detecting step ofdetecting a first reference parameter of the controlled object otherthan the controlled variable of the controlled object; a secondreference parameter-detecting step of detecting a second referenceparameter of the controlled object other than the controlled variableand the first reference parameter of the controlled object; a targetvalue-setting step of setting a target value serving as a target towhich the controlled variable is controlled; and a controlinput-calculating step of calculating a first input value forfeedforward control of the controlled variable, according to the firstreference parameter and the second reference parameter, using acorrelation model representative of a correlation between the firstreference parameter, the second reference parameter, and the first inputvalue, calculating a second input value for performing feedback controlof the controlled variable such that the controlled variable is causedto converge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein said control input-calculatingstep comprises: an error parameter-calculating step of calculating anerror parameter indicative of a control error to be compensated for bythe first input value, based on the controlled variable and the targetvalue; a modification value-calculating step of calculating amodification value for modifying the correlation model according to theerror parameter; a first influence degree parameter-calculating step ofcalculating a first influence degree parameter indicative of a degree ofinfluence of the first reference parameter on the error parameter, usinga first influence degree model representative of a correlation betweenthe first influence degree parameter and the first reference parameter;a corrected modification value-calculating step of calculating acorrected modification value by correcting the modification value by thefirst influence degree parameter; a model-modifying step of modifyingthe correlation model according to the corrected modification value; anda first input value-calculating step of calculating the first inputvalue using the modified correlation model.
 15. A method as claimed inclaim 14, further comprising: a second influence degreeparameter-calculating step of calculating a second influence degreeparameter indicative of a degree of influence of the second referenceparameter on the error parameter, using a second influence degree modelrepresentative of a correlation between the second influence degreeparameter and the second reference parameter; and a corrected errorparameter-calculating step of calculating a corrected error parameter bycorrecting the error parameter by the second influence degree parameter;wherein said modification value-calculating step includes calculatingthe modification value according to the corrected error parameter.
 16. Amethod as claimed in claim 15, wherein the predetermined feedbackcontrol algorithm is an algorithm to which is applied a predeterminedfirst response-specifying control algorithm for specifying a convergencerate of a difference between the controlled variable and the targetvalue to 0, wherein said modification value-calculating step includescalculating the modification value with an algorithm to which is applieda predetermined second response-specifying control algorithm thatspecifies a convergence rate of the corrected error parameter to 0, andwherein in the predetermined second response-specifying controlalgorithm, the convergence rate of the corrected error parameter to 0 isset such that it becomes lower than the convergence rate of thedifference to 0 in the predetermined first response-specifying controlalgorithm.
 17. A method as claimed in claim 14, wherein the controlledobject is an internal combustion engine in which an amount of intake airdrawn into a cylinder of the engine is changed by a variable intakemechanism, as desired, the controlled variable being an air-fuel ratioof a mixture in the engine, the control input being an amount of fuel tobe supplied to the engine, the second reference parameter being anoperating condition parameter indicative of an operating condition ofthe variable intake mechanism.
 18. A method as claimed in claim 14,wherein the controlled object is a vehicle using the engine as a drivesource thereof, the controlled variable being a wheel speed of thevehicle, the control input being an output of the engine, the secondreference parameter being one of a limit value of the output of theengine and a rotational speed of the engine.
 19. An engine control unitincluding a control program for causing a computer to execute a methodof controlling a controlled variable of a controlled object by a controlinput: wherein the control program causes the computer to detect thecontrolled variable; detect a reference parameter of the controlledobject other than the controlled variable of the controlled object; seta target value serving as a target to which the controlled variable iscontrolled; and calculate a first input value for feedforward control ofthe controlled variable, according to the reference parameter, using acorrelation model representative of a correlation between the referenceparameter and the first input value, calculate a second input value forperforming feedback control of the controlled variable such that thecontrolled variable is caused to converge to the target value, with apredetermined feedback control algorithm, and calculate the controlinput based on the first input value and the second input value, whereinwhen causing the computer to calculate the control input, the controlprogram causes the computer to calculate an error parameter indicativeof a control error to be compensated for by the first input value, basedon the controlled variable and the target value; calculate an influencedegree parameter indicative of a degree of influence of the referenceparameter on the error parameter by using an influence degree modelrepresentative of a correlation between the influence degree parameterand the reference parameter; calculate a corrected error parameter bycorrecting the error parameter by the influence degree parameter; modifythe correlation model according to the corrected error parameter; andcalculate the first input value using the modified correlation model.20. An engine control unit as claimed in claim 19, wherein thepredetermined feedback control algorithm is an algorithm to which isapplied a predetermined first response-specifying control algorithm thatspecifies a convergence rate of a difference between the controlledvariable and the target value to 0, wherein when causing the computer tomodify the correlation model, the control program causes the computer tocalculate a modification value with an algorithm to which is applied apredetermined second response-specifying control algorithm thatspecifies a convergence rate of the corrected error parameter to 0, andmodify the correlation model by the modification value, and wherein inthe predetermined second response-specifying control algorithm, theconvergence rate of the corrected error parameter to 0 is set such thatit becomes lower than the convergence rate of the difference to 0 in thepredetermined first response-specifying control algorithm.
 21. An enginecontrol unit as claimed in claim 19, wherein the controlled object is aninternal combustion engine in which an amount of intake air drawn into acylinder of the engine is changed by a variable intake mechanism, asdesired, the controlled variable being an air-fuel ratio of a mixture inthe engine, the control input being an amount of fuel to be supplied tothe engine, the reference parameter being an operating conditionparameter indicative of an operating condition of the variable intakemechanism.
 22. An engine control unit as claimed in claim 19, whereinthe controlled object is a vehicle using the engine as a drive sourcethereof, the controlled variable being a wheel speed of the vehicle, thecontrol input being an output of the engine, the reference parameterbeing one of a limit value of the output of the engine and a rotationalspeed of the engine.
 23. An engine control unit including a controlprogram for causing a computer to execute a method of controlling acontrolled variable of a controlled object by a control input: whereinthe control program causes the computer to detect the controlledvariable; detect a first reference parameter of the controlled objectother than the controlled variable of the controlled object; detect asecond reference parameter of the controlled object other than thecontrolled variable and the first reference parameter of the controlledobject; set a target value serving as a target to which the controlledvariable is controlled; and calculate a first input value forfeedforward control of the controlled variable, according to the firstreference parameter and the second reference parameter, using acorrelation model representative of a correlation between the firstreference parameter, the second reference parameter, and the first inputvalue, calculating a second input value for performing feedback controlof the controlled variable such that the controlled variable is causedto converge to the target value, with a predetermined feedback controlalgorithm, and calculating the control input based on the first inputvalue and the second input value, wherein when causing the computer tocalculate the control input, the control program causes the computer tocalculate an error parameter indicative of a control error to becompensated for by the first input value, based on the controlledvariable and the target value; calculate a modification value formodifying the correlation model according to the error parameter;calculate a first influence degree parameter indicative of a degree ofinfluence of the first reference parameter on the error parameter, usinga first influence degree model representative of a correlation betweenthe first influence degree parameter and the first reference parameter;calculate a corrected modification value by correcting the modificationvalue by the first influence degree parameter; modify the correlationmodel according to the corrected modification value; and calculate thefirst input value using the modified correlation model.
 24. An enginecontrol unit as claimed in claim 23, wherein the control program furthercauses the computer to calculate a second influence degree parameterindicative of a degree of influence of the second reference parameter onthe error parameter, using a second influence degree modelrepresentative of a correlation between the second influence degreeparameter and the second reference parameter; and calculate a correctederror parameter by correcting the error parameter by the secondinfluence degree parameter; wherein when causing the computer tocalculate the modification value, the control program causes thecomputer to calculate the modification value according to the correctederror parameter.
 25. An engine control unit as claimed in claim 24,wherein the predetermined feedback control algorithm is an algorithm towhich is applied a predetermined first response-specifying controlalgorithm for specifying a convergence rate of a difference between thecontrolled variable and the target value to 0, wherein when causing thecomputer to calculate the modification value, the control program causesthe computer to calculate the modification value with an algorithm towhich is applied a predetermined second response-specifying controlalgorithm that specifies a convergence rate of the corrected errorparameter to 0, and wherein in the predetermined secondresponse-specifying control algorithm, the convergence rate of thecorrected error parameter to 0 is set such that it becomes lower thanthe convergence rate of the difference to 0 in the predetermined firstresponse-specifying control algorithm.
 26. An engine control unit asclaimed in claim 23, wherein the controlled object is an internalcombustion engine in which an amount of intake air drawn into a cylinderof the engine is changed by a variable intake mechanism, as desired, thecontrolled variable being an air-fuel ratio of a mixture in the engine,the control input being an amount of fuel to be supplied to the engine,the second reference parameter being an operating condition parameterindicative of an operating condition of the variable intake mechanism.27. An engine control unit as claimed in claim 23, wherein thecontrolled object is a vehicle using the engine as a drive sourcethereof, the controlled variable being a wheel speed of the vehicle, thecontrol input being an output of the engine, the second referenceparameter being one of a limit value of the output of the engine and arotational speed of the engine.